Why does my USB cable have an extra red plug?

"The black connection provides power and data. The red one is power only. The second cord should be used if the black one doesn't provide enough power to power and/or charge the device. You'll see a lot of external hard drives come with the same type of plug (though usually both are black).

EDIT: To answer your second question, plug the device in using the black cable. If it runs normally and charges (assuming this has a battery), then you're fine. If, however, the device doesn't operate correctly or doesn't charge, then you will need to plug the red cord into another USB port. You wouldn't cause any harm to the device by plugging in both cables. Honestly, the red one would even be plugged into a different computer since it's just power only."

A USB port can only provide up to 500 mA. The device that cable came with requires more than that, so it must be connected to two USB ports at the same time to get enough power.

"You don't need both cables plugged in, but it'll likely be a lot slower to charge and may default to usb 1.1 for data as well due to the lower power provided by only one cable.

My Nook does something similar, except instead of using the two cords, it has an extra long connector in the device end so that it'll only work on the nook and will use full power when you use their wall adapter. You can still plug in standard usb to it, but it takes forever to charge then."