The storage controller 250 may incorporate: a CPU 251 for executing programs stored in memory 252; a Memory 252 for storing programs and data; a Cache 253 for storing data received from the top tier storage apparatus and read from the HDD; a FCIF 255 for connecting to the top tier storage apparatus 100; a SATA IF 257 for connecting to the HDDs 201 (if HDD has another interface such as SCSI, SAS, a