A storage device includes a secondary power source, a charging circuit, a protection circuit and a main system. The secondary power source includes a plurality of capacitors, is charged based on a charging voltage, and generates an internal power supply voltage. The charging circuit generates the charging voltage based on an external power supply voltage. The protection circuit monitors whether at least one of the plurality of capacitors is defective, and blocks at least one defective capacitor. The main system operates based on the external or internal power supply voltage. The protection circuit includes a plurality of resistors, a plurality of transistors and a control circuit. The control circuit monitors whether the at least one of the plurality of capacitors is defective using the plurality of resistors and a plurality of currents associated with the plurality of capacitors, and blocks the at least one defective capacitor using the plurality of transistors and a plurality of control signals.