a data i / o system comprises a plurality of storage devices and a controller which controls the storage devices . in the data i / o system , the controller further includes a read / write unit , responsive to the subsequent receipt of a read request and a write request , for reading data stored in the storage devices and writing data in the storage devices , a logical volume management unit for mapping between a logical image of the data storage of a host processor and an actual space in the storage devices , a volume management unit for managing an active primary production volume and second multiple mirror volumes created as mirror images of the primary volume , and an s - vol restoring unit for restoring the data of a first s - vol with the data of a second s - vol depending on the type of an error that happens in the first s - vol .