(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 . (2004-06-17)
