The invention provides a data storage device comprising a controller and a data storage unit. The data storage unit comprises a first system storage area and a second system storage area. The first system storage area stores an original operating system, and comprises a first initial sector address. The second system storage area stores a backup operating system, and comprises a second initial sector address. The controller comprises a firmware. A boot pointer offset procedure is defined in the firmware. A boot pointer is preset by the firmware to point to the first initial sector address of the first system storage area, and therefore a boot operation is executed by the original operating system. When the original operating system damages, the firmware executes the boot pointer offset procedure to offset the boot pointer to the second initial sector address, and executes the boot operation by the backup operating system.