1. A storage system, comprising: a flash memory for writing data in page units and erasing said data in units of a block configured from a plurality of said pages, and having a plurality of said blocks and requiring the deletion of said blocks containing said pages in order to update said data; a cache memory for writing and temporarily storing data to be written in said flash memory at a speed that is faster than said flash memory; a controller for controlling the reading, writing and deletion of data of said flash memory and the reading and writing of data of said cache memory, and detecting the generation of a defective block in said flash memory; and a host computer for issuing a command requesting the write processing of said data; wherein said controller migrates prescribed data stored in said flash memory to said cache memory upon detecting the generation of said defective block in said flash memory and, even upon receiving from said host computer a command for updating the migrated data, disables the writing of data in said flash memory based on said command.