Storage system that tracks mapping to a memory module to be detached therefrom

A storage system connectable to a host, comprising:

• a plurality of interface units;
• a plurality of semiconductor memory modules, each being detachably coupled with one of the interface units;
• and
• a controller configured to maintain an address conversion table indicating mapping between a logical address and a physical address of a memory region of the semiconductor memory modules, wherein
• when the controller determines that a first semiconductor memory module is to be detached, the controller converts physical addresses of the first semiconductor memory module into corresponding logical addresses using the address conversion table and copies valid data stored in the physical addresses to another semiconductor memory module and updates the address conversion table to indicate new mappings for the corresponding logical addresses of the valid data, and
• after the first semiconductor memory module has been detached from the corresponding interface unit, the controller is further configured to stop data writing to remaining semiconductor memory modules when an over-provisioning ratio is smaller than a predetermined value.