<hardware, storage> (Or "hard disk drive", "hard drive", "floppy disk
drive", "floppy drive") A peripheral device that reads and writes hard disks or
floppy disks. The drive contains a motor to rotate the disk at a constant rate
and one or more read/write heads which are positioned over the desired track by
a servo mechanism. It also contains the electronics to amplify the signals from
the heads to normal digital logic levels and vice versa.
In order for a disk drive to start to read or write a given location a
read/write head must be positioned radially over the right track and
rotationally over the start of the right sector.
Radial motion is known as "seeking" and it is this which causes most of the
intermittent noise heard during disk activity. There is usually one head for
each disk surface and all heads move together. The set of locations which are
accessible with the heads in a given radial position are known as a "cylinder".
The "seek time" is the time taken to seek to a different cylinder.
The disk is constantly rotating (except for some floppy disk drives where the
motor is switched off between accesses to reduce wear and power consumption) so
positioning the heads over the right sector is simply a matter of waiting until
it arrives under the head. With a single set of heads this "rotational latency"
will be on average half a revolution but some big drives have multiple sets of
heads spaced at equal angles around the disk.
If seeking and rotation are independent, access time is seek time + rotational
latency. When accessing multiple tracks sequentially, data is sometimes arranged
so that by the time the seek from one track to the next has finished, the disk
has rotated just enough to begin accessing the next track.
See also sector interleave.
The disks may be removable disks; floppy disks always are, removable hard disks
were common on mainframes and minicomputers but less so on microcomputers until
the mid 1990s(?) with products like the Zip Drive.
A CD-ROM drive is not usually referred to as a disk drive.
Two common interfaces for disk drives (and other devices) are SCSI and IDE.
ST506 used to be common in microcomputers (in the 1980s?).
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