When data are transferred to a cache from disk drives, through a back end I/F unit, a redundant code verifier of a data path controller compares write-history information (a Destaging Counter (DC) value) embedded in a redundant code of the data with a DC value embedded in a redundant code of a corresponding parity, thereby verifying whether the data are imprecise. In addition, in a case where the data are updated, the redundant code verifier makes a similar verification when old data to be used for creating a parity are read. When the data are not imprecise, a redundant code generator of the data path controller adds to new data and the parity a redundant code into which a new DC value has been embedded. When the data are imprecise, a controller informs a host of an occurrence of a problem through a front end I/F unit.