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 Pro File.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.
HDDs became the dominant secondary storage device for general-purpose computers by the early 1960s.
Continuously improved, HDDs have maintained this position into the modern era of servers and personal computers.
The first IBM drive, the 350 RAMAC in 1956, was approximately the size of two medium-sized refrigerators and stored five million six-bit characters (3.75 megabytes), the 1301 used an array of heads, one per platter, moving as a single unit.
Cylinder-mode read/write operations were supported, and the heads flew about 250 micro-inches (about 6 µm) above the platter surface.
Performance is specified by the time required to move the heads to a track or cylinder (average access time) plus the time it takes for the desired sector to move under the head (average latency, which is a function of the physical rotational speed in revolutions per minute), and finally the speed at which the data is transmitted (data rate).
The two most common form factors for modern HDDs are 3.5-inch, for desktop computers, and 2.5-inch, primarily for laptops.
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.
Motion of the head array depended upon a binary adder system of hydraulic actuators which assured repeatable positioning.