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, one or more battery units, and a drive unit having a plurality of nonvolatile devices the one or more controller units comprising: a control unit; a volatile memory; and a nonvolatile memory device, the one or more power supply units being coupled to the one or more battery units and including an external power input unit and a power supply control unit, wherein the volatile memory includes a read data storage area for storing read data read from a plurality of nonvolatile devices, and a write data storage area for storing write data written from the computer, and when the storage system selects to retain data stored in the volatile memory, a capacity of the write data storage area of the volatile memory is increased or decreased based on a capacity of a battery, wherein, when a power supply from an exterior is interrupted, the one or more controller units determine content of interruption of the power supply from the exterior, and based on a determination result and a data backup mode information set via the management computer, select whether to continue retaining the data stored in the volatile memory or to store the data in the nonvolatile memory device and retain the data therein, and wherein a selection is made as to whether to inhibit restarting of the storage system or to restart the storage system and select a write-through mode for write access to the volatile memory when the power supply from the exterior is resumed.