A host computer is connected with a magnetic disk storage device by an SCSI bus. In the magnetic disk storage device, a plurality of partitions are set in a disk drive unit and have device identifiers (ID's) respectively allocated thereto as SCSI ID's=1, 2 and 3, which are supported by a disk controller. When the host computer has acquired the control of the SCSI bus through arbitration and has selected, for example, the partition with the device identifier SCSI ID=1, the disk controller permits the host computer to access the partition in response to the selection. Since the partitions are different in attributes, properties etc., they appear as magnetic disk storage devices that are separate from one another when viewed from the host computer. Thus, the single magnetic disk storage device can be managed as a plurality of storage devices of different nature.