A data storage device with flash memory. The controller receives a mode selection command from a host. In response to the mode selection command, the controller sends a ready-to-transfer message to the host, to further receive a data out message from the host that is sent by the host in response to the ready-to-transfer message. The ready-to-transfer message and the data out message are UFS protocol information unit (UPIU) messages. The data out message is arranged to rewrite a first mode page setting among a plurality of mode page settings of firmware stored in the flash memory. In response to the data out message, the controller determines whether the data out message will change mode parameters which cannot be rewritten in the first mode page setting, to adopt or refuse new mode parameters issued through the data out message for the first mode page setting.