The invention provides a data storage device. In one embodiment, the data storage device is coupled to a host, and comprises a downgrade memory and a controller. The downgrade memory comprises a plurality of blocks, wherein each of the blocks comprises a plurality of pages, each of the pages comprises a plurality of sectors, and some of the sectors are defect sectors. The controller generates a defect table for recording a plurality of defect addresses of the defect sectors in the blocks, receives a plurality of data sectors to be written to the downgrade memory from the host, determines a plurality of first physical sector addresses for storing the data sectors according to the defect table, and sends write commands to the downgrade memory to direct the downgrade memory to write the data sectors to the downgrade memory according to the first physical sector addresses.