A data storage device executes sector guarantee processing at an unexpected power supply disconnection. The sector guarantee processing can be accurately performed even if the fluctuation of power supply voltage during normal operation is high. The voltage of the power supply is monitored via an A/D converter. The control unit calculates a voltage change value, which is a relative value of the detected voltage of the power supply, and judges power supply disconnection by the level of the voltage change value, to perform sector guarantee processing. The voltage fluctuation during normal operation and the voltage fluctuation at power supply disconnection can be distinguished, power supply disconnection can be accurately detected, and write disable status can be judged.