A FLASH memory is used in data storage and is further stored with a logical-to-physical address mapping table and a write protection mapping table. The write protection mapping table shows the write protection statuses of the different logical addresses. In accordance with logical addresses issued via a dynamic capacity management command from a host, a controller of the data storage device modifies the logical-to-physical address mapping table to break the logical-to-physical mapping relationship of the issued logical addresses. Further, the controller asserts a flag, corresponding to the issued logical addresses, in the write protection mapping table, to a write protected mode. According to a change in the amount of write-protected flags of the write protection mapping table, the controller adjusts an end-of-life judgment value of the FLASH memory and thereby a lifespan of the FLASH memory is prolonged.