When a bus is used as a data communication channel, data within a disk unit cannot be reproduced or copied into a spare disk while a control unit is making read/write processing based on a request from a host computer, or vice versa. Thus, a loop is constructed by a fiber channel capable of time division multiplex function, and the processing between the disk unit and the spare is performed not through the control unit so that the data within the disk can be copied into the spare while the processing between the host computer and the disk unit is being performed, or both processing operations can be executed in parallel.