The present invention provides a data-storage device. The data-storage device includes a flash memory and a controller. The flash memory has a plurality of blocks and each of the blocks has a plurality of pages. The blocks include a plurality of bad blocks that are labeled as damaged. The controller selects one of the bad blocks as a test block, and reads the pages in the test block to determine whether the pages in the test block are damaged. When all the pages in the test block are undamaged, the controller labels the test block as a spare block.