I've encountered what may be a related problem with several (8) RT21's usb connected. It seems that at least Win7 boot can't properly assign multiple USB devices that are already powered up. I delved
I have 9 rt-21's and 2 gh base wireless transmitters on one asus computer running win 8.1, the rt-21's are always turned on before the computer by my power sequencer, the wireless boxes are usb power