Glossary
SUBSTRATE – The material the disk platter is made of beneath the magnetic coating. Hard disks are generally made of aluminum or magnesium alloy (or glass, for optical disks) while the substrate of floppies is usually mylar.
SURFACE – The top or bottom side of the platter which is coated with the magnetic material for recording data. On some drives one surface may be reserved for positioning information.
T
THIN FILM – A type of coating, used for disk surfaces. Thin film surfaces allow more bits to be stored per disk.
TPI – Acronym for tracks per inch. The number of tracks or cylinders that are written in each inch of travel across the surface of a disk.
TRACK – One of the many concentric magnetic circle patterns written on a disk surface as a guide to where to store and read the data.
TRACK DENSITY – How closely the tracks are packed on a disk surface. The number is specified as tracks per inch (TPI).
TRACK TO TRACK SEEK TIME – The time required for the read/write heads to move to an adjacent track.
TRANSFER RATE – The rate at which the disk sends and receives data from the controller. Drive specifications usually reference a high number that is the burst mode rate for transferring data across the interface from the disk buffer to system RAM. Sustained data transfer is at a much lower rate because of system processing overhead, head switches, and seeks.
U
UNFORMATTED CAPACITY – The total number of bytes of data that could be fit onto a disk. Formatting the disk requires some of this space to record location, boundary definitions, and timing information. After formatting, user data can be stored on the remaining disk space, known as formatted capacity. The size of a Maxtor drive is expressed in formatted capacity.
V
VOICE COIL – A type of motor used to move the disk read/write head in and out to the right track.
W
WEDGE SERVO – The position on every track that contains data used by the closed loop positioning control. This information is used to fine tune the position of the read/write heads exactly over the track center.
WINCHESTER DISKS – Hard disks that use a technology similar to an IBM model using Winchester as the code name. These disks use read/write heads that ride just above the magnetic surface, held up by the air flow created by the turning disk. When the disk stops turning, the heads land on the surface, which has a specially lubricated coating. Winchester disks must be sealed and have a filtration system since ordinary dust particles are large enough to catch between the head and the disk.
WRITE ONCE – In the context of optical disks, technologies that allow the drive to store data on a disk and read it back, but not to erase it.