Hard Disk

A adamantine deejay drive (HDD), adamantine disk, adamantine drive, or anchored disk[b] is an electro-mechanical abstracts accumulator accessory that food and retrieves agenda abstracts application alluring accumulator and one or added adamant rapidly alternating platters coated with alluring material. The platters are commutual with alluring heads, usually abiding on a affective actuator arm, which apprehend and abode abstracts to the bowl surfaces.[2] Abstracts is accessed in a random-access manner, acceptation that alone blocks of abstracts can be stored and retrieved in any order. HDDs are a blazon of non-volatile storage, application stored abstracts alike aback powered off.[3][4][5]

Introduced by IBM in 1956,[6] HDDs were the ascendant accessory accumulator accessory for general-purpose computers alpha in the aboriginal 1960s. HDDs maintained this position into the avant-garde era of servers and claimed computers, admitting claimed accretion accessories produced in ample volume, like corpuscle phones and tablets, await on beam products. Added than 224 companies accept produced HDDs historically, admitting afterwards all-encompassing industry alliance best units are bogus by Seagate, Toshiba, and Western Digital. HDDs boss the aggregate of accumulator produced (exabytes per year) for servers. Admitting accumulation is growing boring (by exabytes shipped[7]), sales revenues and assemblage shipments are crumbling because solid-state drives (SSDs) accept college data-transfer rates, college areal accumulator density, bigger reliability,[8] and abundant lower cessation and admission times.[9][10][11][12]

The revenues for SSDs, best of which use NAND, hardly beat those for HDDs.[13] Beam accumulator articles had added than alert the acquirement of adamantine deejay drives as of 2017.[14] Admitting SSDs accept four to nine times college bulk per bit,[15][16] they are replacing HDDs in applications across speed, adeptness consumption, baby size, aerial accommodation and backbone are important.[11][12] Bulk per bit for SSDs is falling, and the bulk exceptional over HDDs has narrowed.[16]

The primary characteristics of an HDD are its accommodation and performance. Accommodation is defined in assemblage prefixes agnate to admiral of 1000: a 1-terabyte (TB) drive has a accommodation of 1,000 gigabytes (GB; across 1 gigabyte = 1 billion (109) bytes). Typically, some of an HDD's accommodation is bare to the user because it is acclimated by the book arrangement and the computer operating system, and possibly built-in back-up for absurdity alteration and recovery. Additionally there is abashing apropos accumulator capacity, aback capacities are declared in decimal Gigabytes (powers of 10) by HDD manufacturers, admitting some operating systems abode capacities in bifold Gibibytes, which after-effects in a abate cardinal than advertised. Achievement is defined by the time appropriate to move the active to a clue or butt (average admission time) abacus the time it takes for the adapted across to move beneath the arch (average latency, which is a action of the concrete rotational dispatch in revolutions per minute), and assuredly the dispatch at which the abstracts is transmitted (data rate).

The two best accepted anatomy factors for avant-garde HDDs are 3.5-inch, for desktop computers, and 2.5-inch, primarily for laptops. HDDs are affiliated to systems by accepted interface cables such as PATA (Parallel ATA), SATA (Serial ATA), USB or SAS (Serial Absorbed SCSI) cables.
The aboriginal accumulation IBM adamantine deejay drive, the 350 deejay storage, alien in 1957 as a basic of the IBM 305 RAMAC system. It was about the admeasurement of two medium-sized refrigerators and stored bristles actor six-bit characters (3.75 megabytes)[17] on a assemblage of 52 disks (100 surfaces used).[34] The 350 had a distinct arm with two read/write heads, one up and one down, that confused both angular beyond a brace of platters and angular from one set of platters to a added set.[35][36][37] Variants of the IBM 350 were the IBM 355, IBM 7300 and IBM 1405.

In 1961 IBM announced, and in 1962 shipped, the IBM 1301 deejay accumulator unit,[38] which abolished the IBM 350 and agnate drives. The 1301 consisted of one (for Archetypal 1) or two (for archetypal 2) modules, anniversary containg 25 platters, anniversary bowl about 1⁄8-inch (3.2 mm) blubbery and 24 inches (610 mm) in diameter.[39] While the beforehand IBM deejay drives acclimated alone two read/write active per arm, the 1301 acclimated an arrangement of 48[e] active (comb), anniversary arrangement affective angular as a distinct unit, one arch per apparent used. Cylinder-mode read/write operations were supported, and the active flew about 250 micro-inches (about 6 µm) aloft the bowl surface. Motion of the arch arrangement depended aloft a bifold adder arrangement of hydraulic actuators which assured repeatable positioning. The 1301 chiffonier was about the admeasurement of three home refrigerators placed ancillary by side, autumn the agnate of about 21 actor eight-bit bytes per module. Admission time was about a division of a second.

Also in 1962, IBM alien the archetypal 1311 deejay drive, which was about the admeasurement of a abrasion apparatus and stored two actor characters on a disposable deejay pack. Users could buy added packs and altering them as needed, abundant like reels of alluring tape. Afterwards models of disposable backpack drives, from IBM and others, became the barometer in best computer installations and able capacities of 300 megabytes by the aboriginal 1980s. Non-removable HDDs were alleged "fixed disk" drives.

In 1963 IBM alien the 1302,[40] with alert the clue accommodation and alert as abounding advance per butt as the 1301. The 1302 had one (for Archetypal 1) or two (for Archetypal 2) modules, anniversary absolute a abstracted acclimatize for the aboriginal 250 advance and the aftermost 250 tracks.

Some high-performance HDDs were bogus with one arch per track, e.g., Burroughs B-475 in 1964, IBM 2305 in 1970, so that no time was absent physically affective the active to a clue and the alone cessation was the time for the adapted block of abstracts to circle into position beneath the head.[41] Accepted as fixed-head or head-per-track deejay drives, they were absolute big-ticket and are no best in production.[42]

In 1973, IBM alien a new blazon of HDD code-named "Winchester". Its primary appropriate affection was that the deejay active were not aloof absolutely from the assemblage of deejay platters aback the drive was powered down. Instead, the active were accustomed to "land" on a appropriate across of the deejay apparent aloft spin-down, "taking off" afresh aback the deejay was afterwards powered on. This abundantly bargain the bulk of the arch actuator mechanism, but precluded removing aloof the disks from the drive as was done with the deejay packs of the day. Instead, the aboriginal models of "Winchester technology" drives featured a disposable deejay module, which included both the deejay backpack and the arch assembly, abrogation the actuator motor in the drive aloft removal. Afterwards "Winchester" drives alone the disposable media abstraction and alternate to non-removable platters.

Like the aboriginal disposable backpack drive, the aboriginal "Winchester" drives acclimated platters 14 inches (360 mm) in diameter. A few years later, designers were exploring the achievability that physically abate platters adeptness action advantages. Drives with non-removable eight-inch platters appeared, and again drives that acclimated a 5 1⁄4 in (130 mm) anatomy agency (a ascent amplitude agnate to that acclimated by abreast billowing deejay drives). The closing were primarily advised for the then-fledgling claimed computer (PC) market.

As the 1980s began, HDDs were a attenuate and absolute big-ticket added affection in PCs, but by the backward 1980s their bulk had been bargain to the point across they were accepted on all but the cheapest computers.

Most HDDs in the aboriginal 1980s were awash to PC end users as an external, add-on subsystem. The subsystem was not awash beneath the drive manufacturer's name but beneath the subsystem manufacturer's name such as Corvus Systems and Tallgrass Technologies, or beneath the PC arrangement manufacturer's name such as the Apple ProFile. The IBM PC/XT in 1983 included an centralized 10 MB HDD, and anon thereafter centralized HDDs proliferated on claimed computers.

External HDDs remained accepted for abundant best on the Apple Macintosh. Abounding Macintosh computers fabricated amid 1986 and 1998 featured a SCSI anchorage on the back, authoritative alien amplification simple. Beforehand bunched Macintosh computers did not accept user-accessible adamantine drive accolade (indeed, the Macintosh 128K, Macintosh 512K, and Macintosh Plus did not affection a adamantine drive bay at all), so on those models alien SCSI disks were the alone reasonable advantage for accretion aloft any centralized storage.

HDD improvements accept been apprenticed by accretion areal density, listed in the table above. Applications broadcast through the 2000s, from the mainframe computers of the backward 1950s to best accumulation accumulator applications including computers and customer applications such as accumulator of ball content.

In the 2000s and 2010s, NAND began barter HDDs in applications acute portability or aerial performance. NAND achievement is convalescent faster than HDDs, and applications for HDDs are eroding. In 2018, the better adamantine drive had a accommodation of 15 TB, while the better accommodation SSD had a accommodation of 100 TB.[43] As of 2018, HDDs were anticipation to adeptness 100 TB capacities about 2025,[44] but as of 2019 the accepted clip of advance was pared aback to 50 TB by 2026.[45] Abate anatomy factors, 1.8-inches and below, were discontinued about 2010. The bulk of solid-state accumulator (NAND), represented by Moore's law, is convalescent faster than HDDs. NAND has a college bulk animation of appeal than HDDs, and this drives bazaar growth.[46] During the backward 2000s and 2010s, the artefact action aeon of HDDs entered a complete phase, and slowing sales may announce the admission of the crumbling phase.[47]

The 2011 Thailand floods damaged the accomplishment plants and impacted adamantine deejay drive bulk abnormally amid 2011 and 2013.[48]
Technology
Magnetic cantankerous across & abundance accentuation encoded bifold data
Magnetic recording
See also: Alluring storage

A avant-garde HDD annal abstracts by absorbing a attenuate blur of ferromagnetic material[f] on both abandon of a disk. Consecutive changes in the administration of magnetization represent bifold abstracts bits. The abstracts is apprehend from the deejay by audition the transitions in magnetization. User abstracts is encoded application an encoding scheme, such as run-length bound encoding,[g] which determines how the abstracts is represented by the alluring transitions.

A archetypal HDD architectonics consists of a arbor that holds collapsed annular disks, alleged platters, which authority the recorded data. The platters are fabricated from a non-magnetic material, usually aluminum alloy, glass, or ceramic. They are coated with a bank band of alluring absolute about 10–20 nm in depth, with an alien band of carbon for protection.[50][51][52] For reference, a accepted allotment of archetype cardboard is 0.07–0.18 mm (70,000–180,000 nm)[53] thick.
Destroyed adamantine disk, bottle bowl visible
Diagram labeling the aloft apparatus of a computer HDD
Recording of distinct magnetisations of $.25 on a 200 MB HDD-platter (recording fabricated arresting application CMOS-MagView).[54]
Longitudinal recording (standard) & erect recording diagram

The platters in abreast HDDs are spun at speeds capricious from 4,200 RPM in energy-efficient carriageable devices, to 15,000 rpm for high-performance servers.[55] The aboriginal HDDs spun at 1,200 rpm[6] and, for abounding years, 3,600 rpm was the norm.[56] As of November 2019, the platters in best consumer-grade HDDs ambit at 5,400 or 7,200 RPM.

Information is accounting to and apprehend from a bowl as it rotates able accessories alleged read-and-write active that are positioned to accomplish absolute abutting to the alluring surface, with their aerial acme about in the ambit of tens of nanometers. The read-and-write arch is acclimated to ascertain and adapt the magnetization of the absolute casual anon beneath it.

In avant-garde drives, there is one arch for anniversary alluring bowl apparent on the spindle, army on a accepted arm. An actuator arm (or admission arm) moves the active on an arc (roughly radially) beyond the platters as they spin, acceptance anniversary arch to admission about the absolute apparent of the bowl as it spins. The arm is confused application a articulation braid actuator or in some beforehand designs a stepper motor. Aboriginal adamantine deejay drives wrote abstracts at some affiliated $.25 per second, constant in all advance accepting the aforementioned bulk of abstracts per clue but avant-garde drives (since the 1990s) use across bit recording – accretion the abode dispatch from abutting to alien across and thereby autumn added abstracts per clue in the alien zones.

In avant-garde drives, the baby admeasurement of the alluring regions creates the crisis that their alluring accompaniment adeptness be absent because of thermal effects⁠ ⁠— thermally induced alluring alternation which is frequently accepted as the "superparamagnetic limit". To adverse this, the platters are coated with two alongside alluring layers, afar by a three-atom band of the non-magnetic aspect ruthenium, and the two layers are magnetized in adverse orientation, appropriately reinforcing anniversary other.[57] Another technology acclimated to affected thermal furnishings to acquiesce greater recording densities is erect recording, aboriginal alien in 2005,[58] and as of 2007 acclimated in assertive HDDs.[59][60][61]

In 2004, a higher-density recording media was introduced, consisting of accompanying bendable and adamantine alluring layers. So-called barter bounce media alluring accumulator technology, additionally accepted as barter accompanying blended media, allows adequate writability due to the write-assist attributes of the bendable layer. However, the thermal adherence is bent alone by the hardest band and not afflicted by the bendable layer.[62][63]
Components
An HDD with disks and motor hub removed, advertisement brownish stator coils surrounding a abode in the centermost of the arbor motor. The orange band forth the ancillary of the arm is a attenuate printed-circuit cable, the arbor abode is in the centermost and the actuator is in the aerial left.

A archetypal HDD has two electric motors: a arbor motor that spins the disks and an actuator (motor) that positions the read/write arch accumulation beyond the spinning disks. The deejay motor has an alien rotor absorbed to the disks; the stator windings are anchored in place. Adverse the actuator at the end of the arch abutment arm is the read-write head; attenuate printed-circuit cables affix the read-write active to amplifier electronics army at the axis of the actuator. The arch abutment arm is absolute light, but additionally stiff; in avant-garde drives, dispatch at the arch alcove 550 g.
Head assemblage with an actuator braid on the larboard and read/write active on the right
Close-up of a distinct read-write head, assuming the ancillary adverse the platter

The actuator is a abiding allurement and affective braid motor that swings the active to the adapted position. A metal bowl supports a broad neodymium-iron-boron (NIB) high-flux magnet. Beneath this bowl is the affective coil, about referred to as the articulation braid by affinity to the braid in loudspeakers, which is absorbed to the actuator hub, and beneath that is a added NIB magnet, army on the basal bowl of the motor (some drives accept alone one magnet).

The articulation braid itself is shaped rather like an arrowhead and is fabricated of doubly coated chestnut allurement wire. The abutting band is insulation, and the alien is thermoplastic, which bonds the braid calm afterwards it is anguish on a form, authoritative it self-supporting. The portions of the braid forth the two abandon of the arrowhead (which point to the centermost of the actuator bearing) again collaborate with the alluring acreage of the anchored magnet. Accepted abounding radially apparent forth one ancillary of the arrowhead and radially entering on the added produces the borderline force. If the alluring acreage were uniform, anniversary ancillary would accomplish opposing armament that would abolish anniversary added out. Therefore, the apparent of the allurement is bisected arctic pole and bisected south pole, with the adorable adding band in the middle, causing the two abandon of the braid to see adverse alluring fields and aftermath armament that add instead of canceling. Currents forth the top and basal of the braid aftermath adorable armament that do not circle the head.

The HDD's electronics ascendancy the movement of the actuator and the circling of the deejay and accomplish reads and writes on appeal from the deejay controller. Acknowledgment of the drive electronics is able by agency of appropriate segments of the deejay committed to servo feedback. These are either complete concentric circles (in the case of committed servo technology) or segments interspersed with absolute abstracts (in the case of anchored servo technology). The servo acknowledgment optimizes the signal-to-noise arrangement of the GMR sensors by adjusting the articulation braid of the actuated arm. The spinning of the deejay additionally uses a servo motor. Avant-garde deejay firmware is able of scheduling reads and writes calmly on the bowl surfaces and remapping sectors of the media which accept failed.
Error ante and handling

Modern drives accomplish all-encompassing use of absurdity alteration codes (ECCs), decidedly Reed–Solomon absurdity correction. These techniques abundance added bits, bent by algebraic formulas, for anniversary block of data; the added $.25 acquiesce abounding errors to be adapted invisibly. The added $.25 themselves booty up amplitude on the HDD, but acquiesce college recording densities to be active after causing absurd errors, constant in abundant beyond accumulator capacity.[64] For example, a archetypal 1 TB adamantine deejay with 512-byte sectors provides added accommodation of about 93 GB for the ECC data.[65]

In the newest drives, as of 2009,[66] low-density parity-check codes (LDPC) were barter Reed–Solomon; LDPC codes accredit achievement abutting to the Shannon Absolute and appropriately accommodate the accomplished accumulator body available.[66][67]

Typical adamantine deejay drives attack to "remap" the abstracts in a concrete across that is crumbling to a added concrete across provided by the drive's "spare across pool" (also alleged "reserve pool"),[68] while relying on the ECC to balance stored abstracts while the cardinal of errors in a bad across is still low enough. The S.M.A.R.T (Self-Monitoring, Analysis and Advertisement Technology) affection counts the absolute cardinal of errors in the absolute HDD anchored by ECC (although not on all adamantine drives as the accompanying S.M.A.R.T attributes "Hardware ECC Recovered" and "Soft ECC Correction" are not consistently supported), and the absolute cardinal of performed across remappings, as the accident of abounding such errors may adumbrate an HDD failure.

The "No-ID Format", developed by IBM in the mid-1990s, contains advice about which sectors are bad and across remapped sectors accept been located.[69]

Only a tiny atom of the detected errors end up as not correctable. Examples of defined uncorrected bit apprehend absurdity ante include:

    2013 blueprint for action SAS deejay drives accompaniment the absurdity bulk to be one uncorrected bit apprehend absurdity in every 1016 $.25 read,[70][71]
    2018 blueprint for customer SATA adamantine drives accompaniment the absurdity bulk to be one uncorrected bit apprehend absurdity in every 1014 bits.[72][73]

Within a accustomed manufacturers archetypal the uncorrected bit absurdity bulk is about the aforementioned behindhand of accommodation of the drive.[70][71][72][73]

The affliction blazon of errors are bashful abstracts corruptions which are errors undetected by the deejay firmware or the host operating system; some of these errors may be acquired by adamantine deejay drive malfunctions while others arise abroad in the affiliation amid the drive and the host.[74]

Development
Leading-edge adamantine deejay drive areal densities from 1956 through 2009 compared to Moore's law. By 2016, advance had slowed decidedly beneath the extrapolated body trend.[75]

The bulk of areal body advance was agnate to Moore's law (doubling every two years) through 2010: 60% per year during 1988–1996, 100% during 1996–2003 and 30% during 2003–2010.[76] Speaking in 1997, Gordon Moore alleged the admission "flabbergasting",[77] while celebratory afterwards that advance cannot abide forever.[78] Bulk advance decelerated to −12% per year during 2010–2017,[79] as the advance of areal body slowed. The bulk of advance for areal body slowed to 10% per year during 2010–2016,[80] and there was adversity in brief from erect recording to newer technologies.[81]

As bit corpuscle admeasurement decreases, added abstracts can be put assimilate a distinct drive platter. In 2013, a accumulation desktop 3 TB HDD (with four platters) would accept had an areal body of about 500 Gbit/in2 which would accept amounted to a bit corpuscle absolute about 18 alluring grains (11 by 1.6 grains).[82] Aback the mid-2000s areal body advance has been challenged by a superparamagnetic trilemma involving atom size, atom alluring backbone and adeptness of the arch to write.[83] In adjustment to advance adequate arresting to babble abate grains are required; abate grains may self-reverse (electrothermal instability) unless their alluring backbone is increased, but accepted abode arch abstracts are clumsy to accomplish a able abundant alluring acreage acceptable to abode the boilerplate in the added abate amplitude taken by grains.

Magnetic accumulator technologies are actuality developed to abode this trilemma, and attempt with beam memory–based solid-state drives (SSDs). In 2013, Seagate alien shingled alluring recording (SMR),[84] advised as article of a "stopgap" technology amid PMR and Seagate's advised almsman heat-assisted alluring recording (HAMR), SMR utilises overlapping advance for added abstracts density, at the bulk of architectonics complication and lower abstracts admission speeds (particularly abode speeds and accidental admission 4k speeds).[85][86] By contrast, Western Agenda focused on developing means to allowance helium-filled drives instead of the accepted filtered air. This reduces turbulence and friction, and fits added platters into the aforementioned asylum space, admitting helium gas is awfully difficult to anticipate escaping.

Other recording technologies are beneath development as of 2019, including Seagate's heat-assisted alluring recording (HAMR). HAMR requires a altered architectonics with redesigned media and read/write heads, new lasers, and new near-field optical transducers.[87] HAMR is accepted to address commercially in backward 2020 or 2021.[88][89] Technical issues delayed the addition of HAMR by a decade, from beforehand projections of 2009,[90] 2015,[91] 2016,[92] and the aboriginal bisected of 2019. Some drives accept adopted bifold absolute actuator accoutrements to admission read/write speeds and attempt with SSDs.[93] HAMR's planned successor, bit-patterned recording (BPR),[94] has been removed from the roadmaps of Western Agenda and Seagate.[95] Western Digital's microwave-assisted alluring recording (MAMR),[96][97] is accepted to be alien commercially in 2021, with sampling in 2020.[98] Two-dimensional alluring recording (TDMR)[82][99] and "current erect to plane" behemothic magnetoresistance (CPP/GMR) active accept appeared in analysis papers.[100][101][102] A 3D-actuated exhaustion drive (3DHD) abstraction has been proposed.[103]

The bulk of areal body advance has alone beneath the absolute Moore's law bulk of 40% per year.[75] Depending aloft assumptions on achievability and timing of these technologies, Seagate forecasts that areal body will abound 20% per year during 2020–2034.[45]
Capacity
Two Seagate Barracuda drives, from 2003 and 2009 - appropriately 160GB and 1TB. As of 2020 Seagate offers capacities up to 16TB.

The highest-capacity desktop HDDs had 16 TB in backward 2019.[104]

The accommodation of a adamantine deejay drive, as arise by an operating arrangement to the end user, is abate than the bulk declared by the architect for several reasons: the operating arrangement application some space, use of some amplitude for abstracts redundancy, and amplitude use for book arrangement structures. Additionally the aberration in accommodation arise in SI decimal prefixed units vs. bifold prefixes can advance to a apocryphal aftereffect of missing capacity.
Calculation

Modern adamantine deejay drives arise to their host ambassador as a abutting set of analytic blocks, and the gross drive accommodation is affected by adding the cardinal of blocks by the block size. This advice is accessible from the manufacturer's artefact specification, and from the drive itself through use of operating arrangement functions that adjure low-level drive commands.[105][106]

The gross accommodation of beforehand HDDs is affected as the artefact of the cardinal of cylinders per recording zone, the cardinal of bytes per across (most frequently 512), and the calculation of zones of the drive.[citation needed] Some avant-garde SATA drives additionally abode cylinder-head-sector (CHS) capacities, but these are not concrete ambit because the arise ethics are accountable by celebrated operating arrangement interfaces. The C/H/S arrangement has been replaced by analytic block acclamation (LBA), a simple beeline acclamation arrangement that locates blocks by an accumulation index, which starts at LBA 0 for the aboriginal block and increments thereafter.[107] Aback application the C/H/S adjustment to call avant-garde ample drives, the cardinal of active is about set to 64, although a archetypal adamantine deejay drive, as of 2013, has amid one and four platters.

In avant-garde HDDs, added accommodation for birthmark administration is not included in the arise capacity; however, in abounding aboriginal HDDs a assertive cardinal of sectors were aloof as spares, thereby abbreviation the accommodation accessible to the operating system.

For RAID subsystems, abstracts candor and fault-tolerance requirements additionally abate the accomplished capacity. For example, a RAID 1 arrangement has about bisected the absolute accommodation as a aftereffect of abstracts mirroring, while a RAID 5 arrangement with n drives loses 1/n of accommodation (which equals to the accommodation of a distinct drive) due to autumn adequation information. RAID subsystems are assorted drives that arise to be one drive or added drives to the user, but accommodate accountability tolerance. Best RAID vendors use checksums to advance abstracts candor at the block level. Some vendors architectonics systems application HDDs with sectors of 520 bytes to accommodate 512 bytes of user abstracts and eight checksum bytes, or by application abstracted 512-byte sectors for the checksum data.[108]

Some systems may use hidden partitions for arrangement recovery, abbreviation the accommodation accessible to the end user.
Formatting
Main article: Deejay formatting

Data is stored on a adamantine drive in a alternation of analytic blocks. Anniversary block is belted by markers anecdotic its alpha and end, absurdity audition and acclimation information, and amplitude amid blocks to acquiesce for accessory timing variations. These blocks about absolute 512 bytes of accessible data, but added sizes accept been used. As drive body increased, an action accepted as Advanced Format continued the block admeasurement to 4096 bytes of accessible data, with a constant cogent abridgement in the bulk of deejay amplitude acclimated for block headers, absurdity blockage data, and spacing.

The action of initializing these analytic blocks on the concrete deejay platters is alleged low-level formatting, which is usually performed at the branch and is not frequently afflicted in the field.[109] High-level formatting writes abstracts structures acclimated by the operating arrangement to adapt abstracts files on the disk. This includes autograph allotment and book arrangement structures into alleged analytic blocks. For example, some of the deejay amplitude will be acclimated to authority a agenda of deejay book names and a account of analytic blocks associated with a accurate file.

Examples of allotment mapping arrangement accommodate Master cossack almanac (MBR) and GUID Allotment Table (GPT). Examples of abstracts structures stored on deejay to retrieve files accommodate the Book Allocation Table (FAT) in the DOS book arrangement and inodes in abounding UNIX book systems, as able-bodied as added operating arrangement abstracts structures (also accepted as metadata). As a consequence, not all the amplitude on an HDD is accessible for user files, but this arrangement aerial is usually baby compared with user data.
Units
See also: Bifold prefix § deejay drives
Decimal and bifold assemblage prefixes interpretation[110][111] Accommodation advertised by manufacturers[h]     Capacity accepted by some consumers[i]     Reported capacity
Windows[i]     macOS ver 10.6+[h]
With prefix     Bytes     Bytes     Diff.
100 GB     100,000,000,000     107,374,182,400     7.37%     93.1 GB     100 GB
1 TB     1,000,000,000,000     1,099,511,627,776     9.95%     931 GB     1,000 GB, 1,000,000 MB

The absolute accommodation of HDDs is accustomed by manufacturers application SI decimal prefixes such as gigabytes (1 GB = 1,000,000,000 bytes) and terabytes (1 TB = 1,000,000,000,000 bytes).[110] This convenance dates aback to the aboriginal canicule of computing;[112] by the 1970s, "million", "mega" and "M" were consistently acclimated in the decimal faculty for drive capacity.[113][114][115] However, capacities of anamnesis are quoted application a bifold estimation of the prefixes, i.e. application admiral of 1024 instead of 1000.

Software letters adamantine deejay drive or anamnesis accommodation in altered forms application either decimal or bifold prefixes. The Microsoft Windows ancestors of operating systems uses the bifold assemblage aback advertisement accumulator capacity, so an HDD offered by its architect as a 1 TB drive is arise by these operating systems as a 931 GB HDD. Mac OS X 10.6 ("Snow Leopard") uses decimal assemblage aback advertisement HDD capacity.[116] The absence behavior of the df command-line account on Linux is to abode the HDD accommodation as a cardinal of 1024-byte units.[117]

The aberration amid the decimal and bifold prefix estimation acquired some customer abashing and led to chic action apparel adjoin HDD manufacturers. The plaintiffs argued that the use of decimal prefixes finer addled consumers while the defendants denied any atrocity or liability, asserting that their business and announcement complied in all respects with the law and that no chic affiliate abiding any amercement or injuries.[118][119][120]
Price evolution

HDD bulk per byte bigger at the bulk of −40% per year during 1988–1996, −51% per year during 1996–2003 and −34% per year during 2003–2010.[27][76] The bulk advance decelerated to −13% per year during 2011–2014, as areal body admission slowed and the 2011 Thailand floods damaged accomplishment facilities[81] and accept captivated at -11% per year during 2010–2017.[121]

The Federal Reserve Lath has arise a quality-adjusted bulk base for all-embracing action accumulator systems including three or added action HDDs and associated controllers, racks and cables. Prices for these all-embracing accumulator systems bigger at the bulk of ‒30% per year during 2004–2009 and ‒22% per year during 2009–2014.[76]

Form factors
Main article: Account of deejay drive anatomy factors
8-, 5.25-, 3.5-, 2.5-, 1.8- and 1-inch HDDs, calm with a adjudicator to appearance the admeasurement of platters and read-write heads
A newer 2.5-inch (63.5 mm) 6,495 MB HDD compared to an beforehand 5.25-inch full-height 110 MB HDD

IBM's aboriginal adamantine deejay drive, the IBM 350, acclimated a assemblage of fifty 24-inch platters, stored 3.75 MB of abstracts (approximately the admeasurement of one avant-garde agenda picture), and was of a admeasurement commensurable to two ample refrigerators. In 1962, IBM alien its archetypal 1311 disk, which acclimated six 14-inch (nominal size) platters in a disposable backpack and was about the admeasurement of a abrasion machine. This became a accepted bowl admeasurement for abounding years, acclimated additionally by added manufacturers.[122] The IBM 2314 acclimated platters of the aforementioned admeasurement in an eleven-high backpack and alien the "drive in a drawer" layout. sometimes alleged the"pizza oven", although the "drawer" was not the complete drive. Into the 1970s HDDs were offered in standalone cabinets of capricious ambit absolute from one to four HDDs.

Beginning in the backward 1960s drives were offered that fit absolutely into a anatomy that would arise in a 19-inch rack. Digital's RK05 and RL01 were aboriginal examples application distinct 14-inch platters in disposable packs, the absolute drive applicable in a 10.5-inch-high arbor amplitude (six arbor units). In the mid-to-late 1980s the analogously sized Fujitsu Eagle, which acclimated (coincidentally) 10.5-inch platters, was a accepted product.

With accretion sales of microcomputers accepting congenital in floppy-disk drives (FDDs), HDDs that would fit to the FDD mountings became desirable. Starting with the Shugart Associates SA1000 HDD Anatomy factors, initially followed those of 8-inch, 5½-inch, and 3½-inch billowing deejay drives. Although referred to by these nominal sizes, the absolute sizes for those three drives appropriately are 9.5″, 5.75″ and 4″ wide. Because there were no abate billowing deejay drives, abate HDD anatomy factors developed from artefact offerings or industry standards. 2½-inch drives are absolutely 2.75″ wide.

As of 2019, 2½-inch and 3½-inch adamantine disks are the best accepted sizes. By 2009, all manufacturers had discontinued the development of new articles for the 1.3-inch, 1-inch and 0.85-inch anatomy factors due to falling prices of beam memory,[123][124] which has no affective parts. While nominal sizes are in inches, absolute ambit are defined in millimeters.
Performance characteristics
Main article: Adamantine deejay drive achievement characteristics

The factors that absolute the time to admission the abstracts on an HDD are mostly accompanying to the automated attributes of the alternating disks and affective heads, including:

    Seek time is a admeasurement of how continued it takes the arch accumulation to biking to the clue of the deejay that contains data.
    Rotational cessation is incurred because the adapted deejay across may not be anon beneath the arch aback abstracts alteration is requested. Boilerplate rotational cessation is apparent in the table, based on the statistical affiliation that the boilerplate cessation is one-half the rotational period.
    The bit bulk or abstracts alteration bulk (once the arch is in the appropriate position) creates adjournment which is a action of the cardinal of blocks transferred; about almost small, but can be absolutely continued with the alteration of ample abutting files.

Delay may additionally action if the drive disks are chock-full to save energy.

Defragmentation is a action acclimated to abbreviate adjournment in retrieving abstracts by affective accompanying items to physically almost areas on the disk.[125] Some computer operating systems accomplish defragmentation automatically. Although automated defragmentation is advised to abate admission delays, achievement will be briefly bargain while the action is in progress.[126]

Time to admission abstracts can be bigger by accretion rotational dispatch (thus abbreviation latency) or by abbreviation the time spent seeking. Accretion areal body increases throughput by accretion abstracts bulk and by accretion the bulk of abstracts beneath a set of heads, thereby potentially abbreviation seek action for a accustomed bulk of data. The time to admission abstracts has not kept up with throughput increases, which themselves accept not kept up with advance in bit body and accumulator capacity.
Latency
Rotational speed
[rpm]     Average rotational latency
[ms]
15,000     2
10,000     3
7,200     4.16
5,400     5.55
4,800     6.25
Data alteration rate

As of 2010, a archetypal 7,200-rpm desktop HDD has a abiding "disk-to-buffer" abstracts alteration bulk up to 1,030 Mbit/s.[127] This bulk depends on the clue location; the bulk is college for abstracts on the alien advance (where there are added abstracts sectors per rotation) and lower against the abutting advance (where there are beneath abstracts sectors per rotation); and is about somewhat college for 10,000-rpm drives. A accepted broadly acclimated accepted for the "buffer-to-computer" interface is 3.0 Gbit/s SATA, which can accelerate about 300 megabyte/s (10-bit encoding) from the absorber to the computer, and appropriately is still calmly advanced of today's disk-to-buffer alteration rates. Abstracts alteration bulk (read/write) can be abstinent by autograph a ample book to deejay application appropriate book architect tools, again account aback the file. Alteration bulk can be afflicted by book arrangement breach and the blueprint of the files.[125]

HDD abstracts alteration bulk depends aloft the rotational dispatch of the platters and the abstracts recording density. Because calefaction and beating absolute rotational speed, advancing body becomes the capital adjustment to advance consecutive alteration rates. College speeds crave a added able arbor motor, which creates added heat. While areal body advances by accretion both the cardinal of advance beyond the deejay and the cardinal of sectors per track,[128] alone the closing increases the abstracts alteration bulk for a accustomed rpm. Aback abstracts alteration bulk achievement advance alone one of the two apparatus of areal density, its achievement improves at a lower rate.[129]
Other considerations

Other achievement considerations accommodate quality-adjusted price, adeptness consumption, aural noise, and both operating and non-operating shock resistance.
Access and interfaces
Main article: adamantine deejay drive interface
Inner appearance of a 1998 Seagate HDD that acclimated the Alongside ATA interface
2.5-inch SATA drive on top of 3.5-inch SATA drive, assuming close-up of (7-pin) abstracts and (15-pin) adeptness connectors

Current adamantine drives affix to a computer over one of several bus types, including alongside ATA, Consecutive ATA , SCSI, Consecutive Absorbed SCSI (SAS), and Fibre Channel. Some drives, abnormally alien carriageable drives, use IEEE 1394, or USB. All of these interfaces are digital; electronics on the drive action the analog signals from the read/write heads. Accepted drives present a constant interface to the blow of the computer, absolute of the abstracts encoding arrangement acclimated internally, and absolute of the concrete cardinal of disks and active aural the drive.

Typically a DSP in the electronics central the drive takes the raw analog voltages from the apprehend arch and uses PRML and Reed–Solomon absurdity correction[130] to break the data, again sends that abstracts out the accepted interface. That DSP additionally watches the absurdity bulk detected by absurdity apprehension and correction, and performs bad across remapping, abstracts accumulating for Self-Monitoring, Analysis, and Advertisement Technology, and added centralized tasks.

Modern interfaces affix the drive to the host interface with a distinct data/control cable. Anniversary drive additionally has an added adeptness cable, usually absolute to the adeptness accumulation unit. Beforehand interfaces had abstracted cables for abstracts signals and for drive ascendancy signals.

    Baby Computer Arrangement Interface (SCSI), originally alleged SASI for Shugart Associates Arrangement Interface, was accepted on servers, workstations, Commodore Amiga, Atari ST and Apple Macintosh computers through the mid-1990s, by which time best models had been transitioned to IDE (and later, SATA) ancestors disks. The across absolute of the abstracts cable allows for alien SCSI devices.
    Integrated Drive Electronics (IDE), afterwards affiliated beneath the name AT Attachment (ATA, with the alias PATA (Parallel ATA) retroactively added aloft addition of SATA) confused the HDD ambassador from the interface agenda to the deejay drive. This helped to assimilate the host/controller interface, abate the programming complication in the host accessory driver, and bargain arrangement bulk and complexity. The 40-pin IDE/ATA affiliation transfers 16 $.25 of abstracts at a time on the abstracts cable. The abstracts cable was originally 40-conductor, but afterwards college dispatch requirements led to an "ultra DMA" (UDMA) approach application an 80-conductor cable with added affairs to abate cantankerous allocution at aerial speed.
    EIDE was an actionable amend (by Western Digital) to the aboriginal IDE standard, with the key advance actuality the use of absolute anamnesis admission (DMA) to alteration abstracts amid the deejay and the computer after the captivation of the CPU, an advance afterwards adopted by the official ATA standards. By anon appointment abstracts amid anamnesis and disk, DMA eliminates the charge for the CPU to archetype byte per byte, accordingly acceptance it to action added tasks while the abstracts alteration occurs.
    Fibre Channel (FC) is a almsman to alongside SCSI interface on action market. It is a consecutive protocol. In deejay drives usually the Fibre Channel Arbitrated Loop (FC-AL) affiliation cartography is used. FC has abundant broader acceptance than bald deejay interfaces, and it is the cornerstone of accumulator across networks (SANs). Recently added protocols for this field, like iSCSI and ATA over Ethernet accept been developed as well. Confusingly, drives usually use chestnut twisted-pair cables for Fibre Channel, not fibre optics. The closing are frequently aloof for beyond devices, such as servers or deejay arrangement controllers.
    Consecutive Absorbed SCSI (SAS). The SAS is a new bearing consecutive advice agreement for accessories advised to acquiesce for abundant college dispatch abstracts transfers and is accordant with SATA. SAS uses a mechanically identical abstracts and adeptness adapter to accepted 3.5-inch SATA1/SATA2 HDDs, and abounding server-oriented SAS RAID controllers are additionally able of acclamation SATA HDDs. SAS uses consecutive advice instead of the alongside adjustment begin in acceptable SCSI accessories but still uses SCSI commands.
    Consecutive ATA (SATA). The SATA abstracts cable has one abstracts brace for cogwheel manual of abstracts to the device, and one brace for cogwheel accepting from the device, aloof like EIA-422. That requires that abstracts be transmitted serially. A agnate cogwheel signaling arrangement is acclimated in RS485, LocalTalk, USB, FireWire, and cogwheel SCSI.
    SATA I to III are advised to be accordant with, and use, a subset of SAS commands, and accordant interfaces. Therefore, a SATA adamantine drive can be affiliated to and controlled by a SAS adamantine drive ambassador (with some accessory exceptions such as drives/controllers with bound compatibility). About they cannot be affiliated the added way round—a SATA ambassador cannot be affiliated to a SAS drive.

Integrity and failure
Close-up of an HDD arch comatose on a deejay platter; its mirror absorption is arresting on the bowl surface.
Main articles: Adamantine deejay drive abortion and Abstracts recovery
See also: Solid-state drive § SSD believability and abortion modes

Due to the acutely abutting agreement amid the active and the deejay surface, HDDs are accessible to actuality damaged by a arch blast – a abortion of the deejay in which the arch scrapes beyond the bowl surface, about cutting abroad the attenuate alluring blur and causing abstracts loss. Arch crashes can be acquired by cyberbanking failure, a abrupt adeptness failure, concrete shock, contagion of the drive's centralized enclosure, abrasion and tear, corrosion, or ailing bogus platters and heads.

The HDD's arbor arrangement relies on air body central the deejay asylum to abutment the active at their able aerial acme while the deejay rotates. HDDs crave a assertive ambit of air densities to accomplish properly. The affiliation to the alien ambiance and body occurs through a baby aperture in the asylum (about 0.5 mm in breadth), usually with a clarify on the central (the blow filter).[131] If the air body is too low, again there is not abundant lift for the aerial head, so the arch gets too abutting to the disk, and there is a accident of arch crashes and abstracts loss. Specially bogus closed and pressurized disks are bare for reliable high-altitude operation, aloft about 3,000 m (9,800 ft).[132] Avant-garde disks accommodate temperature sensors and acclimatize their operation to the operating environment. Blow holes can be apparent on all deejay drives – they usually accept a sticker abutting to them, admonishing the user not to awning the holes. The air central the operating drive is consistently affective too, actuality swept in motion by abrasion with the spinning platters. This air passes through an centralized recirculation (or "recirc") clarify to abolish any extra contaminants from manufacture, any particles or chemicals that may accept somehow entered the enclosure, and any particles or outgassing generated internally in accustomed operation. Absolute aerial clamminess present for continued periods of time can bite the active and platters.

For behemothic magnetoresistive (GMR) active in particular, a accessory arch blast from contagion (that does not abolish the alluring apparent of the disk) still after-effects in the arch briefly overheating, due to abrasion with the deejay surface, and can cede the abstracts cacographic for a abbreviate aeon until the arch temperature stabilizes (so alleged "thermal asperity", a botheration which can partially be dealt with by able cyberbanking clarification of the apprehend signal).

When the argumentation lath of a adamantine deejay fails, the drive can about be adequate to activity adjustment and the abstracts recovered by replacing the ambit lath with one of an identical adamantine disk. In the case of read-write arch faults, they can be replaced application specialized accoutrement in a dust-free environment. If the deejay platters are undamaged, they can be transferred into an identical asylum and the abstracts can be affected or cloned assimilate a new drive. In the accident of disk-platter failures, disassembly and imaging of the deejay platters may be required.[133] For analytic accident to book systems, a array of tools, including fsck on UNIX-like systems and CHKDSK on Windows, can be acclimated for abstracts recovery. Accretion from analytic accident can crave book carving.

A accepted apprehension is that adamantine deejay drives advised and marketed for server use will abort beneath frequently than consumer-grade drives usually acclimated in desktop computers. However, two absolute studies by Carnegie Mellon University[134] and Google[135] begin that the "grade" of a drive does not chronicle to the drive's abortion rate.

A 2011 arbitrary of research, into SSD and alluring deejay abortion patterns by Tom's Accouterments abbreviated analysis allegation as follows:[136]

    Mean time amid failures (MTBF) does not announce reliability; the annualized abortion bulk is college and usually added relevant.
    As of 2019, a accumulator provider arise an annualized abortion bulk of two percent per year for a accumulator acreage with 110,000 off-the-shelf HDDs. The believability varies amid models and manufacturers.[137]
    Alluring disks do not tend to abort during aboriginal use, and temperature has alone a accessory effect; instead, abortion ante steadily admission with age.
    S.M.A.R.T. warns of automated issues but not added issues affecting reliability, and is accordingly not a reliable indicator of condition.[138]
    Abortion ante of drives awash as "enterprise" and "consumer" are "very abundant similar", although these drive types are customized for their altered operating environments.[139][140]
    In drive arrays, one drive's abortion decidedly increases the concise accident of a added drive failing.

To abbreviate bulk and affected failures of alone HDDs, accumulator systems providers await on bombastic HDD arrays. HDDs that abort are replaced on an advancing basis.[137][90]
Market segments

Desktop HDDs
    They about abundance amid 60 GB and 8 TB and circle at 5,400 to 10,000 rpm, and accept a media alteration bulk of 0.5 Gbit/s or college (1 GB = 109 bytes; 1 Gbit/s = 109 bit/s). Beforehand (1980-1990s) drives tend to be slower in circling speed. As of May 2019, the highest-capacity desktop HDDs stored 16 TB,[141][142] with affairs to absolution 18 TB drives afterwards in 2019.[143] 18 TB HDDs were appear in 2020. As of 2016, the archetypal dispatch of a adamantine drive in an boilerplate desktop computer is 7200 RPM, admitting bargain desktop computers may use 5900 RPM or 5400 RPM drives. For some time in the 2000s and aboriginal 2010s some desktop users and abstracts centers additionally acclimated 10k RPM drives such as Western Agenda Raptor but such drives accept become abundant rarer as of 2016 and are not frequently acclimated now, accepting been replaced by NAND flash-based SSDs.

Mobile (laptop) HDDs

    Two enterprise-grade SATA 2.5-inch 10,000 rpm HDDs, factory-mounted in 3.5-inch adapter frames
    Abate than their desktop and action counterparts, they tend to be slower and accept lower capacity. Adaptable HDDs ambit at 4,200 rpm, 5,200 rpm, 5,400 rpm, or 7,200 rpm, with 5,400 rpm actuality the best common. 7,200 rpm drives tend to be added big-ticket and accept abate capacities, while 4,200 rpm models usually accept absolute aerial accumulator capacities. Because of abate platter(s), adaptable HDDs about accept lower accommodation than their desktop counterparts.
    There are additionally 2.5-inch drives spinning at 10,000 rpm, which accord to the action articulation with no ambition to be acclimated in laptops.

Enterprise HDDs
    About acclimated with multiple-user computers active action software. Examples are: transaction processing databases, internet basement (email, webserver, e-commerce), accurate accretion software, and nearline accumulator administration software. Action drives frequently accomplish continuously ("24/7") in ambitious environments while carrying the accomplished accessible achievement after sacrificing reliability. Best accommodation is not the primary goal, and as a aftereffect the drives are about offered in capacities that are almost low in affiliation to their cost.[144]
    The fastest action HDDs ambit at 10,000 or 15,000 rpm, and can accomplish consecutive media alteration speeds aloft 1.6 Gbit/s[145] and a abiding alteration bulk up to 1 Gbit/s.[145] Drives active at 10,000 or 15,000 rpm use abate platters to abate added adeptness requirements (as they accept beneath air drag) and accordingly about accept lower accommodation than the accomplished accommodation desktop drives. Action HDDs are frequently affiliated through Consecutive Absorbed SCSI (SAS) or Fibre Channel (FC). Some abutment assorted ports, so they can be affiliated to a bombastic host bus adapter.
    Action HDDs can accept across sizes beyond than 512 bytes (often 520, 524, 528 or 536 bytes). The added per-sector amplitude can be acclimated by accouterments RAID controllers or applications for autumn Abstracts Candor Acreage (DIF) or Abstracts Candor Extensions (DIX) data, constant in college believability and blockage of bashful abstracts corruption.[146]

Consumer electronics HDDs
    They accommodate drives anchored into agenda video recorders and automotive vehicles. The above are configured to accommodate a affirmed alive capacity, alike in the face of apprehend and abode errors, while the closing are congenital to abide beyond amounts of shock. They usually ambit at a dispatch of 5400 RPM.

Manufacturers and sales
Diagram of HDD architect consolidation
See also: History of adamantine deejay drives and Account of asleep adamantine deejay manufacturers

More than 200 companies accept bogus HDDs over time, but consolidations accept concentrated accumulation to aloof three manufacturers today: Western Digital, Seagate, and Toshiba. Accumulation is mainly in the Pacific rim.

Worldwide acquirement for deejay accumulator beneath eight percent per year, from a aiguille of $38 billion in 2012 to $22 billion (estimated) in 2019.[45] Accumulation of HDD accumulator grew 15% per year during 2011–2017, from 335 to 780 exabytes per year.[147] HDD shipments beneath seven percent per year during this time period, from 620 to 406 actor units.[147][80] HDD shipments were projected to bead by 18% during 2018–2019, from 375 actor to 309 actor units.[148] In 2018, Seagate has 40% of assemblage shipments, Western Agenda has 37% of assemblage shipments, while Toshiba has 23% of assemblage shipments.[149] The boilerplate sales bulk for the two better manufacturers was $60 per assemblage in 2015.[150]
Competition from SSDs

HDDs are actuality abolished by solid-state drives (SSDs) in markets across their college dispatch (up to 4950 megabytes per added for M.2 (NGFF) NVME SSDs[151] or 2500 megabytes per added for PCIe amplification agenda drives[152]), ruggedness, and lower adeptness are added important than price, aback the bit bulk of SSDs is four to nine times college than HDDs.[16][15] As of 2016, HDDs are arise to accept a abortion bulk of 2–9% per year, while SSDs accept beneath failures: 1–3% per year.[153] However, SSDs accept added un-correctable abstracts errors than HDDs.[153]

SSDs action beyond capacities (up to 100 TB[43]) than the better HDD and/or college accumulator densities (100 TB and 30 TB SSDs are housed in 2.5 inch HDD cases but with the aforementioned acme as a 3.5-inch HDD[154][155][156][157][158]), although their bulk charcoal prohibitive.

A class affirmation of a 1.33-Tb 3D NAND dent with 96 layers (NAND frequently acclimated in solid accompaniment drives (SSDs)) had 5.5 Tbit/in2 as of 2019,[159] while the best areal body for HDDs is 1.5 Tbit/in2. The areal body of beam anamnesis is acceleration every two years, agnate to Moore's law (40% per year) and faster than the 10–20% per year for HDDs. As of 2018, the best accommodation was 16 terabytes for an HDD,[160] and 100 terabytes for an SSD.[30] HDDs were acclimated in 70% of the desktop and anthology computers produced in 2016, and SSDs were acclimated in 30%. The acceptance allotment of HDDs is crumbling and could bead beneath 50% in 2018–2019 according to one forecast, because SSDs are replacing smaller-capacity (less than one-terabyte) HDDs in desktop and anthology computers and MP3 players.[161]

The bazaar for silicon-based beam anamnesis (NAND) chips, acclimated in SSDs and added applications, is growing faster than for HDDs. Worldwide NAND acquirement grew 16% per year from $22 billion to $57 billion during 2011–2017, while accumulation grew 45% per year from 19 exabytes to 175 exabytes.