A device adaptor that controls a disk, wherein a storage area is divided into a plurality of blocks, detects a block that includes a read error. Next, the device adaptor detects a write omission in blocks that are within a predetermined range from the block that includes the read error. To be specific, the device adaptor reads data from the blocks that are in the vicinity of the block that includes the unrecovered read error in a suspect disk. Next, the device adaptor reads from a redundant disk, data of a block that corresponds to the block that includes the unrecovered read error (data of block “B” in an example shown in FIG. 1) and compares the read data to the data read from the suspect disk to detect the write omission.