Pre-update data is copied from a first storage device onto a second storage device in response to an update instruction to update data on the backup target volume on the first storage device. A copy status of each data on the backup target volume is managed with position information of the data mapped thereto. If bad data is present in the data on the backup target volume, the position information indicating the position of the bad data is searched. In accordance with the copy status managed with the position information mapped thereto, it is determined whether the pre-update data of the bad data is stored on the second storage device.