1. A storage system for receiving and processing and I/O request from an external device, comprising: a plurality of flash memory devices (FMDs) comprising a plurality of flash memories; and a controller for accessing an FMD specified by a designated access destination in said received I/O request, from among said plurality of FMDs, wherein two or more parity groups configured of two or more FMDs from among said plurality of FMDs, wherein FMDs with different internal configurations are included in said plurality of FMDs, wherein each parity group is configured of FMDs having an identical internal configuration.