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 (logical volume) and an actual space in the storage devices, a volume management unit for managing an active primary production volume (P-VOL) and second multiple mirror volumes (S-VOL) 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.