Source : WordNet®

magnetic disk
     n : (computer science) a memory device consisting of a flat disk
         covered with a magnetic coating on which information is
         stored [syn: {magnetic disc}, {disk}, {disc}]

Source : Free On-Line Dictionary of Computing

magnetic disk
     
         A flat rotating disc covered on one or both sides
        with some magnetisable material.  The two main types are the
        {hard disk} and the {floppy disk}.
     
        Small areas or "zones" on a magnetic disk are magnetised.  The
        magnetisation is aligned in one of two opposing orientations
        with respect to the recording {head}.  The {head} writes data
        by altering the alignment and reads data by detecting current
        pulses induced in a coil as zones with different magnetic
        alignment pass underneath it.
     
        Data is stored on either or both surfaces of discs in
        concentric rings called "{tracks}".  Each track is divided
        into a whole number of "{sectors}".  Where multiple (rigid)
        discs are mounted on the same axle the set of tracks at the
        same radius on all their surfaces is known as a "{cylinder}".
     
        Data is read and written by a {disk drive} which rotates the
        discs and positions the read/write "{heads}" over the desired
        track(s).  The latter radial movement is known as "{seeking}".
        There is usually one head for each surface that stores data
        though, for increased performance (from reduced {rotational
        latency}), it is possible, though expensive, to position
        multiple heads at equally spaced angles around the discs.
     
        Therefore there are two states that can be detected for each
        zone - a change in alignment, or no change.
     
        Ideally a data bit of one or zero can be recorded in each zone
        of magnetisation, however, if a zero represents an absence of
        magnetic change, the detection of several consecutive zeros
        would imply accurately measuring the length of time of the
        absence, i.e., measuring the time between two magnetic
        changes.  However, the variability of motor speed limits the
        number of consecutive zeros which can be read reliably.
     
        The best recording methods accurately follow the
        characteristics of the magnetic and rotational aspects in
        recording the disk, to be as dense as possible in recording
        bits.
     
        Compare {magnetic drum}, {compact disc}, {optical disk},
        {magneto-optical disk}.
     
        (2003-03-10)