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.
