A data storage device includes a memory device and a memory controller. The memory controller uses a first predetermined memory block as a buffer to receive data from a host device. In response to a write command received from the host device, the memory controller determines a sub-region corresponding to the write command, determines whether the sub-region is a system data sub-region and accordingly determines whether to use a second predetermined memory block as another buffer to receive data from the host device. When the memory controller determines that the sub-region corresponding to the write command is a system data sub-region, the memory controller writes the data into the second predetermined memory block. When the memory controller determines that the sub-region corresponding to the write command is not a system data sub-region, the memory controller writes the data into the first predetermined memory block.