Non-removable HDDs were called "fixed disk" drives.
Some high-performance HDDs were manufactured with one head per track (e.g.
Motion of the head array depended upon a binary adder system of hydraulic actuators which assured repeatable positioning.
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.
HDDs are connected to systems by standard interface cables such as PATA (Parallel ATA), SATA (Serial ATA), USB or SAS (Serial Attached SCSI) cables.
, IBM announced HDDs in 1956 as a component of the IBM 305 RAMAC system and as a new component to enhance the existing IBM 650 system, a general-purpose mainframe.
Data is accessed in a random-access manner, meaning that individual blocks of data can be stored or retrieved in any order and not only sequentially.