A method of data writing for a data storage device includes steps of: determining whether an event of power drop/loss is recorded, wherein the event of power drop/loss is associated with a power supplied by an external device; when it is determined that the event of power drop/loss is recorded, determining whether a backup power source operates abnormally; and when it is determined that the backup power source operates abnormally, the data storage device enters from a normal mode into a write through mode, wherein in the write through mode, data from the external device is written into a buffer area of the data storage device. A data storage device is also provided.