Hard disk drive

A hard disk drive is a physical device that contains hard disk platters, a spindle, read and write heads, read and write arms, electrical motors, integrated electronics and an enclosure.

Using a rigid disk and sealing the unit allows much tighter tolerances that in a floppy disk, consequently hard disks can store much more data than floppy disk and are much faster (at transferring data). In 2001, a typical hard disk might store 30 GB of data, rotate at 10000 revolutions per minute, and have a peak transfer rate of about 10 MB/s.

The disk drive is a type of disk storage that stores and retrieves digital data from a planar magnetic surface. Data is stored or "written" by transmitting an electromagnetic flux through an antenna or "write head" that is very close to a magneticly polarizable material that changes its polarization due to the flux. Detection of the magnet fields on the disk, or "reading" is performed in a reverse manner, as the magnetic fields cause electrical change in the coil or "read head" that passes over it.

A typical hard disk drive design consists of a central axis, i.e., a spindle upon which several hard disk platters spin, each spinning at the same rate of speed. Between each hard disk platter are read and write arms that contain read and write heads. These arms can move randomly either from the outer edge of a hard disk platter toward the center of a hard disk platter or from the center of a hard disk platter toward the outer edge of a hard disk platter. This random movement allows the read and write heads to access all areas of any given hard disk platter.

The integrated electronics control the movement of the read and write arms to specific areas of the hard disk platters and allow the reading and writing of data to occur.

The sealed enclosure contains the sub-systems of a hard disk drive. Containing the sub-systems within a sealed enclosure prevents contaminating substances, e.g., dirt, from coming into contact with the read and write heads. Contamination of the read and write heads can cause a collision, i.e., a "head crash" between a read and write head and a hard disk platter. This kind of contamination can damage the hard disk platter surface.

More to write including transfer speeds, rotational delay, seek time.

Examples of hard disk access buses: ATA, IDE, SCSI