Data recorded in a disk of a hard disk unit is copied to another hard disk unit. A proxy mode, for making the other hard disk unit which has been copied with the data to perform an I/O process to the hard disk unit in place thereof, is performed. Meanwhile, the firmware of the original hard disk unit is updated, and contents of the disk is set up to a latest updated state. Then, the operation in the proxy mode is stopped, and the operation is switched to an operation in a normal mode where the I/O process is directly performed to the original hard disk unit.