A data storage device is provided. The data storage device includes a flash memory and a controller. The flash memory stores a firmware that includes a plurality of mode page settings, and each mode page setting includes a plurality of mode parameters. The controller receives a data out message arranged to rewrite a first mode page setting among the plurality of mode page settings. When determining, based on a reference array, that the data out message will change the mode parameters which cannot be rewritten in the first mode page setting, the controller rejects to change the mode parameters which cannot be rewritten in the first mode page setting. The reference array stores a rewriteable setting for each bit of the first mode page setting.