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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.
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 Pro File.Capacity is specified in unit prefixes corresponding to powers of 1000: a 1-terabyte (TB) drive has a capacity of 1,000 gigabytes (GB; where 1 gigabyte = 1 billion 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.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 home refrigerators placed side by side, storing the equivalent of about 21 million eight-bit bytes. 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.More than 200 companies have produced HDDs historically, though after extensive industry consolidation most current units are manufactured by Seagate, Toshiba, and Western Digital.
HDD unit shipments and sales revenues are declining, though production (exabytes per year) is growing.
Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down.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.
The latter were primarily intended for the then-fledgling personal computer (PC) market.
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.
A few years later, designers were exploring the possibility that physically smaller platters might offer advantages.