PURPOSE: To maintain the reliability of data and to decrease the generation frequency of switching work caused by the generation of an error by inspecting consistency by collating data stored in the two of the principal/sub rotary storage devices.

CONSTITUTION: Data from principal/sub volumes 140A and 140B being subordinates are voluntarily read and performed with compare-check processing in a disk cache controller 120 by utilizing the interval between the requirements of I/O from the side of a channel 110A (B). An error is restored by copying data from the principal volume 140A (sub volume 1440B) on the correct side to the sub volume 140B (principal volume 140A) where the error is generated in accordance with necessity. Thus, the consistancy of data in the principal/sub volumes 140A and 140B can always be maintained and at the same time the generation of a data ghost is previously prevented.