A data storage device includes a flash memory, a voltage detection device, and a controller. The flash memory is arranged to store data. The voltage detection device is arranged to detect a supply voltage received by the data storage device. The controller is configured to receive write commands from a host, and perform a prohibition mode when the supply voltage is outside a predetermined range, wherein the write command is arranged to enable the controller to write the flash memory, and the controller is further configured to disable all of the write commands received from the host in the prohibition mode.