there is realized a storage system which can reduce the limitation of a device number used by a host cpu at the time of duplexing data . a micro processor of a source external storage controller unit unifies a plurality of write data received from the host cpu to the same data bus to provide one unit of data transmitting to a target storage system . a number not dependent on the device number used when the host cpu specifies a device is allocated to this . then , the source external storage controller unit sends the write data to the target external storage controller unit . this can receive / send the data without depending on the target specifying device number .