PURPOSE: To enable preventive maintenance for the restoration of an error physical volume without input to or output from other volumes by logging errors of a disk array, copying the physical volume of the disk array, and incorporating the copied physical volume in the disk array.

CONSTITUTION: If an intermittent fault occurs to one physical volume in logical volumes 1, 2, and 3, the physical volume of the intermittent fault is temporarily shifted to the doubled constitution of a stand-by volume 4 and dynamic copying is performed. For the dynamic copying, copying from a parity volume 3 to the stand-by volume 4 in track units is performed to regard the parity volume 3 as a main volume and the stand-by volume 4 as a subordinate volume. Then the copied stand-by volume 4 is incorporated as a logical volume in the disk array. Consequently, the preventive maintenance for the restoration of the error physical volume can be performed without input to and output from other volumes.