Hard Disk Drive

"Hard drive" redirects here. For other uses, see Hard drive (disambiguation).

Hard disk drive

Video of modern hard disk drive operation (cover removed)
Date invented	24 December 1954[1]
Invented by	IBM team led by Rey Johnson

A hard disk drive (HDD; also hard drive, hard disk, or disk drive)^[2] is a device for storing and retrieving digital information, primarily computer data. It consists of one or more rigid (hence "hard") rapidly rotating discs (often referred to as platters), coated with magnetic material and with magnetic heads arranged to write data to the surfaces and read it from them.
Hard drives are classified as non-volatile, random access, digital, magnetic, data storage devices. Introduced by IBM in 1956, hard disk drives have decreased in cost and physical size over the years while dramatically increasing in capacity and speed.
Hard disk drives have been the dominant device for secondary storage of data in general purpose computers since the early 1960s.^[3] They have maintained this position because advances in their recording capacity, cost, reliability, and speed have kept pace with the requirements for secondary storage.^[3]

History

Main article: History of hard disk drives

Hard disk drives were introduced in 1956 as data storage for an IBM real time transaction processing computer^[4] and were developed for use with general purpose mainframe and mini computers. The first IBM drive, the 350 RAMAC, was approximately the size of two refrigerators and stored 5 million 6-bit characters (the equivalent of 3.75 million 8-bit bytes) on a stack of 50 discs.
In 1961 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.
In 1973, IBM introduced a new type of hard drive codenamed "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 Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down.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.
Like the first removable pack drive, the first "Winchester" drives used platters 14 inches in diameter. A few years later, designers were exploring the possibility that physically smaller platters might offer advantages. Drives with non-removable eight-inch platters appeared, and then drives that fit in a "five and a quarter inch" form factor (a mounting width equivalent to that used by a five and a quarter inch floppy disk drive). The latter were primarily intended for the then-fledgling personal computer market.
As the 1980s began, hard disk drives were a rare and very expensive additional feature on personal computers (PCs); however by the late '80s, their cost had been reduced to the point where they were standard on all but the cheapest PC.
Most hard disk drives in the early 1980s were sold to PC end users as an add on subsystem, not under the drive manufacturer's name but by systems integrators such as the Corvus Disk System or the systems manufacturer such as the Apple ProFile. The IBM PC/XT in 1983 included an internal standard 10MB hard disk drive, and soon thereafter internal hard disk drives proliferated on personal computers.
External hard disk drives remained popular for much longer on the Apple Macintosh. Every Mac made between 1986 and 1998 has a SCSI port on the back, making external expansion easy; also, "toaster" Compact Macs did not have easily accessible hard drive bays (or, in the case of the Mac Plus, any hard drive bay at all), so on those models, external SCSI disks were the only reasonable option.
Driven by areal density doubling every two to four years since their invention, hard disk drives have changed in many ways. A few highlights include:

• Capacity per HDD increasing from 3.75 megabytes^[4] to 3 terabytes or more, about a million times larger.
• Physical volume of HDD decreasing from 68 ft^3[4] or about 2,000 litre (comparable to a large side-by-side refrigerator), to less than 20 ml^[5] (1.2 in³), a 100,000-to-1 decrease.
• Weight decreasing from 2,000 lbs^[4] (~900 kg) to 48 grams^[5] (~0.1 lb), a 20,000-to-1 decrease.
• Price decreasing from about US$15,000 per megabyte^[6] to less than $0.0001 per megabyte ($100/1 terabyte), a greater than 150-million-to-1 decrease.^[7]
• Average access time decreasing from over 100 milliseconds to a few milliseconds, a greater than 40-to-1 improvement.
• Market application expanding from mainframe computers of the late 1950s to most mass storage applications including computers and consumer applications such as storage of entertainment content.