PURPOSE: To exchange a disk volume without stopping a system by rewriting the content of a disk managing table after copying data from the disk volume in which a fault has occured to the one in which no fault occurs, and removing the faulty disk volume.

CONSTITUTION: When the fault occurs in the disk volume 20, a new disk volume 21 is set on a bus 11, and a CPU 1 declares overwrite, and the data is copied from the disk volume 20 in which the fault occurs to the disk volume 21 in which no fault occurs. After the copying of the data being completed, the CPU 1 rewrites the content of the disk managing table 30, and removes the disk 20 in which the fault occurs. Thereby, the disk volumes 20,21 can be exchanged without stopping the system.