I have seen this a few times and it puzzles me. Devices that wont work on the front panel USB headers, but you plug em into the rear USB headers (that attach directly to the motherboard) and they work fine. Yet on those very same front panel usb headers other devices (thumb drives and the like work fine)
The latest example was two external HDD's - neither would work on the front, but were fine on the back. At the same time thumb drives worked fine.
I dont think it was a power issue as both drives have there own power.