In storage subsystems, due to the significant increase in HDD capacity, the time for executing online verification is elongated, affecting accesses from the host computer. By comprehending the status of accesses to the HDD, the sections where error has occurred and the status of restoration thereof, it becomes possible to detect defective or error sections efficiently at an early stage, according to which the reliability and access performance of the storage subsystem can be improved. The present storage subsystem executes one or more of the following processes: (M1) intensive verification of a circumference of an error LBA, (M2) an area-based prioritized verification, and (M3) continuous verification performed for a long period of time to (V1) an area in which error has occurred via IO access, (V2) a highly accessed area, and (V3) during a period of time when IO access is low.