Embodiments of the invention provide a magnetic disk drive having increased efficiency of generation of power for its internal low-voltage blocks. In one embodiment, a magnetic disk drive comprises a main power supply unit for supplying power of a predetermined voltage to a load, the main power supply unit including a switching regulator; a power monitoring circuit for detecting the power supplied from the main power supply unit to the load and for determining whether the detected power exceeds a predetermined power level; and a third power supply unit, if it is determined that the power supplied from the main power supply unit to the load exceeds the predetermined power level, for supplying power of the predetermined voltage to the load, the third power supply unit including a linear regulator.