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For more information on this, take a look at:
Apple Computers and Displays: Powering peripherals through USB
If you have a peripheral (such as a hard drive, audio device, or camera) that is acting unexpectedly—such as not turning on or not being recognized by the computer—the peripheral may require more power than is available. If you have a USB peripheral or a chain of devices, such as a joystick plugged into a keyboard, which requires more than the available power try one of these options:
- Use a USB hub that comes with a power adapter.
- Check that the peripheral offers the ability to connect to more than one USB port at once.
- Find out if the peripheral comes with a power adapter.
- If you cannot confirm your device's use with a hub, connect the device directly to the Apple computer.
Best of luck,