A data storage device includes a data storage medium and a control unit. The control unit is electrically connected to the data storage medium. When switched to a command queue mode, the control unit is configured to receive a queue command comprising at least one task from a host, determine whether the at least one task is a ready-for-execution task, sort at least one ready-for-execution task and selectively reply the host with first queue status information. The first queue status information contains a task quantity corresponding to the at least one ready-for-execution task and at least one task serial number corresponding to the at least one sorted ready-for-execution task.