I had this happen when trying to update my JTV via my X3 Live on the front port of my PC.

When I used the USB interface that came with the guitar, and connected it to the back port of my PC everything worked fine. I dont know exactally which one (if either)

On some computers, front USB ports are nothing more than an unpowered hub. Sometimes these front ports dont supply enough power to certian devices (especially if there is one or more additional devices plugged in) For critical opperations, you should always use rear USB ports just to be sure.

There seem to be lots of reports of updates not working on these forums. Apparently sometimes it takes several tries to get it to work. I dont think anyone has really pinpointed what the problem is, but its a common issue.