A storage disk control unit is provided with a buffer management table that defines correspondence between the sectors of a storage disk and the buffer spaces of the storage disk control unit. Based on the buffer management table, common buffer spaces to be shared by a multiplicity of sectors and individual buffer spaces allocated for other individual sectors are reserved in the buffer means (RAM) of the storage disk control unit. Accordingly, an USB-FDD can perform high-speed read/write operations while minimizing the buffer capacity of the RAM. Further, the buffer spaces reserved for read/write operations can be accommodated in accordance with the use condition of storage disk and the need of the RAM for other purposes.