Powering USB device from arduino uno?
You'd have to test this but there's a chance you could supply the Arduino using a DC power supply on it's 5V pin. In this case DO NOT TURN YPUR SUPPLY HIGHER THAN 5V, as I don't believe there is any protection and will send the full voltage to the USB.  

A safer option would be the Vin pin. Normally you'd want to put 7-12 Volts to this pin but you may want to start at 5V and work your way up just to be safe.

Someone please corrects if I am wrong, but if you never apply more than an external 5V your worst case scenario would be burning out the Arduino but not the USB device.