<storage> (In contrast to floppy disk). One or more rigid magnetic disks
rotating about a central axle with associated read/write heads and electronics,
used to store data. Most hard disks are permanently connected to the drive
(fixed disks) though there are also removable disks.
High speed disks have an access time of 28 milliseconds or less, and low-speed
disks run 65 milliseconds or more. The higher speed disks also transfer their
data faster than the slower speed units.
Each surface of each disk is divided into a number of evenly spaced concentric
circular tracks. The set of all tracks at a given radius on all surfaces (the
tracks which can be accessed without moving the heads) are known as a cylinder.
Each track is divided into sectors.
The heads "float" just above the disk's surface on a current of air. As the disk
turns it moves the air around it, creaing theair current. The head has an
aerodynamic shape so the current pushes it away from the disk. A small spring
pushes the head towards the disk at the same time keeping the head at a constant
distance from the disk (about two microns).
Disk drives are commonly characterised by the kind of interface used to connect
to the computer, e.g. ATA, IDE, SCSI.
See also winchester.
Suchanka's PC-DISK library.
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