A database system which quickly creates a snapshot copy of the backup files. The system has a plurality of database storage subsystems which are normally operating as a mirrored storage system under the control of a mirroring controller. A write suspending unit temporarily suspends write operations to those storage subsystems, and a device detachment unit detaches at least one of the database storage subsystems from the mirrored storage system. A write restarting unit then allows write operations to the remaining database storage subsystems. A correction processor makes corrections to the detached database storage subsystem to resolve any inconsistency in its records, which might have been introduced in relation with such transactions that were in process at the time of the detachment. A data converter converts file location data for the detached database storage subsystem, so that it will serve as a separate database volume.