Andrew,
Apart from the obvious tip of switching the USB port, there is a reported issue about XDS200 emulators and Ubuntu 13.04/64 - not 12.04. Thus I am not 100% sure if this is entirely applicable to your scenario, but you may give this a try.
Unfortunately my Ubuntu 12.04/64 is temporarily out of commission, therefore I am unable to test it myself.
Hope this helps,
Rafael