Data read/write system for a disc storage unit

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.