Thanks guys. Duke, I've done exactly what you've said... at first I screwed up by copying the files out of the (in my case) AVIC5000NEX folder into the root of the USB drive. I learned quickly that you actually have to copy the AVIC5000NEX folder itself onto the root and leave the files inside it alone. I've been trying things that way ever since. I have tried both USB ports with no joy.
SpecTrax, thanks a lot you had me pretty enthusiastic that an older firmware might work. I tried 1.11. I even then downloaded WinRAR (though I had started to use 7zip as an alternative to the built in