A data storage device includes a memory device and a memory controller. The memory device includes multiple memory blocks which include one or more spare memory blocks not written with data and one or more predetermined memory blocks that are configured as a buffer for receiving data from a host device. The memory controller obtains a total number of remaining erasable count of the memory blocks and determines a setting value of a number of said one or more predetermined memory blocks according to a number of currently remaining spare memory block(s), a number of the predetermined memory block(s) that has/have been written with data among said one or more predetermined memory blocks, a predetermined threshold and the total number of remaining erasable count of the memory blocks, and configures the number of the predetermined memory block(s) as the buffer according to the setting value.