A data storage device including a flash memory and a controller. The controller performs a first read operation on the pages of a first block of a first block group, and performs a maintenance process to determine whether the first group read count of the first block group is greater than a read threshold when the first read operation is finished. The controller scans the blocks of the first block group to obtain a plurality of first error bit numbers when the first group read count is greater than the read threshold, and updates the block corresponding to the first error bit number that is greater than an error-bit threshold.