Hard disk drive

A hard disk drive, hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnetic material. The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored and retrieved in any order. HDDs are a type of non-volatile storage, retaining stored data when powered off. Modern HDDs are typically in the form of a small rectangular box, possibly in a disk enclosure for portability.
IBM introduced the first disk drive, the 350 for the 305 RAMAC, in 1956, and disks were the dominant secondary storage device for general-purpose computers beginning in the early 1960s. HDDs maintained this position into the modern era of servers and personal computers, though personal computing devices produced in large volume, like mobile phones and tablets, rely on flash memory storage devices. More than 224 companies have produced HDDs historically, though after extensive industry consolidation, most units are manufactured by Seagate, Toshiba, and Western Digital. HDDs dominate the volume of storage produced for servers. Though production is growing slowly, sales revenues and unit shipments are declining, because solid-state drives have higher data-transfer rates, higher areal storage density, somewhat better reliability, and much lower latency and access times.
The revenues for SSDs, most of which use NAND flash memory, slightly exceeded those for HDDs in 2018. Flash storage products had more than twice the revenue of hard disk drives. Though SSDs have four to nine times higher cost per bit, they are replacing HDDs in applications where speed, power consumption, small size, high capacity and durability are important., the cost per bit of SSDs was falling, and the price premium over HDDs had narrowed.
The primary characteristics of an HDD are its capacity and performance. Capacity is specified in unit prefixes corresponding to powers of : a 1-terabyte drive has a capacity of gigabytes, where 1 gigabyte = 1 000 megabytes = 1 000 000 kilobytes = 1 000 000 000 bytes. Typically, some of an HDD's capacity is unavailable to the user because it is used by the file system and the computer operating system, and possibly inbuilt redundancy for error correction and recovery. There can be confusion regarding storage capacity since capacities are stated in decimal gigabytes by HDD manufacturers, whereas the most commonly used operating systems report capacities in powers of 1024, which results in a smaller number than advertised. Performance is specified as the time required to move the heads to a track or cylinder, the time it takes for the desired sector to move under the head, and finally, the speed at which the data is transmitted.
The two most common form factors for modern HDDs are 3.5-inch, for desktop computers, and 2.5-inch, primarily for laptops and servers. HDDs are connected to systems by standard interface cables such as SATA, USB, SAS, or PATA cables.

History

Parameter	Started with	Improved to	Improvement
Capacity	3.75 megabytes	36 terabytes	9.6-million-to-one
Physical volume			56,000-to-one
Weight			15,000-to-one
Average access time	approx. 600 milliseconds	2.5 ms to 10 ms; RW RAM dependent	about 200-to-one
Price	per megabyte	US$14.4 per terabyte by end of 2022	6.8-billion-to-one
Data density	2,000 bits per square inch	1.4 terabits per square inch in 2023	700-million-to-one
Average lifespan	c. 2000 hrs MTBF	c. 2,500,000 hrs MTBF	1250-to-one

1950s–1960s

The first production IBM hard disk drive, the 350 disk storage, shipped in 1957 as a component of the IBM 305 RAMAC system. It was approximately the size of two large refrigerators and stored five million six-bit characters on a stack of 52 disks. The 350 had a single arm with two read/write heads, one facing up and the other down, that moved both horizontally between a pair of adjacent platters and vertically from one pair of platters to a second set. 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 disk storage unit, which superseded the IBM 350 and similar drives. The 1301 consisted of one or two modules, each containing 25 platters, each platter about thick and in diameter. While the earlier IBM disk drives used only two read/write heads per arm, the 1301 used an array of 48 heads, each array moving horizontally as a single unit, one head per surface used. Cylinder-mode read/write operations were supported, and the heads flew about 250 micro-inches above the platter surface. Motion of the head array depended upon a binary adder system of hydraulic actuators which assured repeatable positioning. The 1301 cabinet was about the size of three large refrigerators placed side by side, storing the equivalent of about 21 million eight-bit bytes per module. Access time was about a quarter of a second.
Also in 1962, IBM introduced the model 1311 disk drive, which was about the size of a washing machine and stored two million characters on a removable disk pack. Users could buy additional packs and interchange them as needed, much like reels of magnetic tape. Later models of removable pack drives, from IBM and others, became the norm in most computer installations and reached capacities of 300 megabytes by the early 1980s. Non-removable HDDs were called "fixed disk" drives.
In 1963, IBM introduced the 1302, with twice the track capacity and twice as many tracks per cylinder as the 1301. The 1302 had one or two modules, each containing a separate comb for the first 250 tracks and the last 250 tracks.
Some high-performance HDDs were manufactured with one head per track, e.g., the Burroughs B-475 in 1964 and the IBM 2305 in 1970, so that no time was lost physically moving the heads to a track and the only latency was the time for the desired block of data to rotate into position under the head. Known as fixed-head or head-per-track disk drives, they were very expensive and are no longer in production.

1970s

In 1973, IBM introduced a new type of HDD code-named "Winchester". Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down. Instead, the heads were allowed to "land" on a special area of the disk surface upon spin-down, "taking off" again when the disk was later powered on. This greatly reduced the cost of the head actuator mechanism but precluded removing just the disks from the drive as was done with the disk packs of the day. Instead, the first models of "Winchester technology" drives featured a removable disk module, which included both the disk pack and the head assembly, leaving the actuator motor in the drive upon removal. Later "Winchester" drives abandoned the removable media concept and returned to non-removable platters.
In 1974, IBM introduced the swinging arm actuator, made feasible because the Winchester recording heads function well when skewed to the recorded tracks. The simple design of the IBM GV drive, invented at IBM's UK Hursley Labs, became IBM's most licensed electro-mechanical invention of all time, the actuator and filtration system being adopted in the 1980s eventually for all HDDs, and still universal nearly 40 years and 10 billion arms later.
Like the first removable pack drive, the first "Winchester" drives used platters in diameter. In 1978, IBM introduced a swing arm drive, the IBM 0680, with eight-inch platters, exploring the possibility that smaller platters might offer advantages. Other eight-inch drives followed, then drives, sized to replace the contemporary floppy disk drives. The latter were primarily intended for the then fledgling personal computer market.

1980s–1990s

Over time, as recording densities were greatly increased, further reductions in disk diameter to 3.5" and 2.5" were found to be optimum. Powerful rare-earth magnet materials became affordable during this period and were complementary to the swing arm actuator design to make possible the compact form factors of modern HDDs.
As the 1980s began, HDDs were a rare and very expensive additional feature in PCs, but by the late 1980s, their cost had been reduced to the point where they were standard on all but the cheapest computers.
Most HDDs in the early 1980s were sold to PC end users as an external, add-on subsystem. The subsystem was not sold under the drive manufacturer's name but under the subsystem manufacturer's name such as Corvus Systems and Tallgrass Technologies, or under the PC system manufacturer's name such as the Apple ProFile. The IBM PC/XT in 1983 included an internal 10 MB HDD, and soon thereafter, internal HDDs proliferated on personal computers.
External HDDs remained popular for much longer on the Apple Macintosh. Many Macintosh computers made between 1986 and 1998 featured a SCSI port on the back, making external expansion simple. Older compact Macintosh computers did not have user-accessible hard drive bays, so on those models, external SCSI disks were the only reasonable option for expanding upon any internal storage.

21st century

HDD improvements have been driven by increasing areal density, listed in the table above. Applications expanded through the 2000s, from the mainframe computers of the late 1950s to most mass storage applications including computers and consumer applications such as storage of entertainment content.
In the 2000s and 2010s, NAND began supplanting HDDs in applications requiring portability or high performance. In 2018, the largest hard drive had a capacity of 15 TB, while the largest capacity SSD had a capacity of 100 TB. In 2018, HDDs were forecast to reach 100 TB capacities around 2025, but, the expected pace of improvement was pared back to 50 TB by 2026. Smaller form factors were discontinued around 2010.
The cost of solid-state storage, represented by Moore's law, is improving faster than HDDs. NAND has a higher price elasticity of demand than HDDs, and this drives market growth. During the late 2000s and 2010s, the product life cycle of HDDs entered a mature phase, and slowing sales may indicate the onset of the declining phase. During the 2020s, NAND adoption rate has significantly accelerated, in no small part due to the rising use of the technology in consumer electronics, while its performance has been improving faster than HDD performance.
The 2011 Thailand floods damaged the manufacturing plants and impacted hard disk drive cost adversely between 2011 and 2013.
In 2019, Western Digital closed its last Malaysian HDD factory due to decreasing demand, to focus on SSD production. All three remaining HDD manufacturers have had decreasing demand for their HDDs since 2014.