A system according to this invention converts a full-copy snapshot into a differential snapshot. The system is composed of a storage subsystem and a server subsystem. The storage subsystem comprises a disk drive and a disk controller. The server subsystem comprises an interface, a processor, and a memory. The disk controller provides storage areas of the disk drive as logical volumes, and stores a differential block bitmap. The processor obtains the differential block bitmap, and identifies a block from the differential block bitmap. The processor obtains, from a full-copy snapshot volume, differential data that is stored in the identified block. The processor stores the obtained differential data in a differential volume. The processor stores, in differential block arrangement information, the location of the differential data stored in the differential volume.