The present invention provides a data storage device including a flash memory and a controller. The flash memory has a plurality of SLC-spare blocks, a plurality of TLC-data blocks and a plurality of TLC-spare blocks. The controller writes a first data sector into a first TLC-spare block, and determines whether a first TLC-data block corresponding to a first logical address has valid data. When the first TLC-data block has valid data, the controller performs a reverse-lookup to obtain a second logical address corresponding to the first TLC-data block, releases the first TLC-data block, a second TLC-data block and a third TLC-data block which are mapped to the second logical address, and maps the first TLC-spare block to the first logical address.