Storage device and memory controller

A storage device, comprising:

• a non-volatile semiconductor memory that has a prescribed unit of write and a unit of block erase which is larger than the prescribed unit of write;
• a memory from which data is readable and to which data is writable;
• and
• a memory controller that performs a read and a write from and to the non-volatile semiconductor memory based on an instruction from an external system,
• wherein a storage area of the non-volatile semiconductor memory includes:
• a scratch block including one or more physical blocks;
• a data block including one or more physical blocks;
• and an erased block including one or more physical blocks,
• wherein the memory has respective tables for managing the scratch block, the data block, and the erased block, and
• wherein the storage device
• writes a data from the external system to a block of the scratch block,
• takes the block of the scratch block as a block of the data block if the block of the scratch block has blank pages fewer than a prescribed number of pages or runs out of blank pages,
• allocates any one of the blocks of the erased block as a new block of the scratch block,
• identifies, from among the blocks of the data block, candidate blocks in which data has been written in all pages thereof, and
• if a number of blocks of the erased block is decreased to a prescribed number or smaller, selects a candidate block having a smallest number of valid data pages only from among the candidate blocks of the data block, copies valid data in the selected candidate block of the data block into the newly-allocated block of the scratch block, and erases the selected candidate block of the data block, wherein selection of the candidate block from among the candidate blocks prevents a block having a blank page from being selected.