I want to know the answer to the following question:
Some devices such as an external hard and optical disk drive can use what?
All the background I'm provided with is that "Such devices can use an external power supply, which is allowed by the standard, or use a dual-input USB cable, one input of which is used for power and data transfer, the other solely for power, which makes the device a non-standard USB device.". Is that enough to answer the question?