Thanks for using Apple Support Communities.
For more information on this, take a look at:
Apple Computers and Displays: Powering peripherals through USB
displays can provide up to 1100 mA at 5 V through the port to which the Apple peripheral or device is connected. This power is available under certain conditions:
If your Apple computer or display is powered off, no power will be provided.
If you have a peripheral 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 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,