Disk Drive | Research & Encyclopedia Articles

Disk Drive

A disk drive is a device used to retrieve and/or store data as part of a computer system's memory. The disk drive operates upon a disk, or platter, which holds the data. All disk drives are electromechanical in nature, meaning that they combine electrical circuitry with moving parts to perform the tasks of data storage and retrieval. Although there are different types of disk drives, they all share some common types of components, namely (1) an electric motor to rotate the disk; (2) a mechanism to store and retrieve data to and from the disk, usually called a read-write head. The read-write head can move from near the center of the disk out to its edge. By spinning the disk and moving the head from center to edge, the read-write head can access any area of the disk; (3) a disk controller, composed of digital circuitry, which performs functions including communicating with the computer's central processing unit (CPU), and directing the read-write head's movement. Since different interfaces exist that connect computer devices (including disk drives), there are different disk controllers. For example, if a disk drive uses an IDE (Integrated Drive Electronics) interface for communication, it will require an IDE controller in order to operate; a SCSI (Small Computer System Interface) requires a SCSI controller; and so forth.

Disk drives are associated with three broad categories of disk technology used to store data: magnetic disks, optical disks, and magneto-optical disks. Magnetic disks consist of a plastic or metal material impregnated with tiny particles that can be magnetized. A read-write head, somewhat similar to the type used on cassette tape recorders, is used by the disk drive to store and retrieve data from the magnetic disk. To write (i.e., store) data to the disk, the magnetic read-write head produces a magnetic field that orients the magnetic particles on the disk's surface. To read data from the disk, the same read-write head registers the magnetic field produced by the particles. Some disk drives are designed to allow removal of the disks that they use. There are several types of removable magnetic disks. Two common types are floppy disks (so-called because the disk is flexible); and removable cartridges, which have a hard outer shell encasing a rigid magnetic disk (or disks). Non-removable magnetic disks contain (like cartridge disks) rigid disks, and are used in hard drives or fixed disk drives. Hard drives usually have multiple disks and multiple read-write heads to maximize capacity and access speed. Hard disk drives and cartridge drives can access their respective data much more quickly than can floppy disk drives.

Optical disc drives are similar to magnetic disk drives in that a rotating disk containing data is read with a device that moves over the disk's surface. However, optical disc drives use optical instead of magnetic technology. Specifically, a laser is employed to read (and sometimes write) the data stored on the discs (optical "disks" are sometimes spelled "discs" to differentiate them from magnetic disks). One type of optical drive is the CD-ROM (compact disk-read only memory) drive, which can read information from CDs, but cannot change it (i.e., a CD-ROM drive cannot write data to a CD). Another type of optical drive is called CD-R (Compact Disc-Recordable), which employs a special type of CD disc upon which data can be recorded once by a CD-R drive. Once data is recorded on the CD-R disc, it can be read by either CD-R or "ordinary" CD-ROM drives. CD-RW (Compact Disc-Rewriteable) drives use special CDs that allow for repeated reading-and-writing. Another optical technology is DVD (Digital Video (or Versatile) Disc). DVD comes in two versions, one of which is computer-compatible, which is called DVD-ROM. A DVD looks just like a CD, but can hold many times more information. A DVD-ROM drive can transfer data roughly 20 times faster than can a CD-ROM drive. Recordable DVD discs and drives (called DVD-R) and rewriteable DVD discs and drives (called DVD-RW) are also available.

Magneto-optical discs are rewritable discs employing a combination of optical and magnetic technology. The read-write head on magneto-optical drives possess both laser and magnetic components. The drive writes data to a disc by using the laser to heat tiny crystals in the disc, which can then be oriented by a magnetic field to encode information.

Disk drives, such as hard drives or CD-ROM drives, are generally available in either internal or external versions. Internal drives are integrated into the computer case itself, whereas an external drive is located outside the computer's case and communicates to the computer through a parallel port. External drives usually cost more than internal drives, have the disadvantage of tying up a parallel port, and may not be able to deliver or receive data as quickly as an internal drive.