a disc storage unit includes discs having surfaces with concentric tracks for storing data in sectors . each sector has identification information including a physical address designating a particular disc surface , sector and track . the physical addresses are contiguous for one surface to another at a first or last sector in a radially outermost or innermost track , respectively . heads are provided for reading and writing data from and onto the discs . the position of each head within a track is determined using servo information recorded between adjacent sectors . address converting circuitry converts logical addresses from an external computer into physical addresses on the discs , including physical addresses which span a plurality of tracks located on both surfaces of a disc , when reading or writing data at logical addresses is commanded . head selecting circuitry responds to the physical addresses by selecting each head corresponding to a surface of the disc on which the physical addresses are located . head position controlling circuitry responds to the physical addresses by positioning each head selected , using the servo information , at each track associated with the physical addresses provided . read / write circuitry reads and writes data from and onto each surface of each disc at the corresponding physical addresses provided .