1. A storage system coupled to a computer or a management computer, the storage system comprising one or more controller units, one or more power supply units and one or more battery units, and a drive unit having a plurality of nonvolatile devices; the controller unit comprising a control unit; a volatile memory; and a nonvolatile memory device; the power supply unit being coupled to the battery unit and including an external power input unit and a power supply control unit; wherein when power supply from an exterior is interrupted, the controller unit determines the content of interruption of power supply from the exterior, and based on the determination result and a data backup mode information set via the management computer, selects whether to continue retaining the data stored in the volatile memory in the volatile memory or to store the data in the nonvolatile memory device and retain the data therein.