a computer system including : disk array system to cause a snapshot corresponding to the selected backup time accessible with the specific address to which the computer can access ; wherein if the snapshot corresponding to the selected backup time is associated with the specific address to which the computer cannot access , then the disk array system making the snapshot accessible with an address to which the computer can access ; and wherein the snapshot relates to differential data between initial backup data and actual backup data at the selected backup time , thereby an acquisition time of the snapshot is different than the selected backup time corresponding to the specific address that can be accessed initially by the computer .