STOP, its NOT a Mobo OR PSU issue.
If you search around, you will find a couple of threads with the same issue.
The issue is because the USB headers carrying current from the mobo to the front cabinet connectors are not able to supply sufficient power (basically lower quality wires) to most devices (especially ones powered directly from USB) that require high power. So pen drives/dongles, connecting cameras/mobiles etc will work fine, but other stuff like external hard drives wont be able to draw sufficient current.
The solution is to use the back USB ports for such high power devices. Alternately, you can try using a separate powered USB hub connected to the front USB connectors. Since the hub is powered, it should be able to supply the devce with sufficient power.
Of course, just connecting to the back connectors is much simpler