I have the same problem. For the original poster the backup seemed to have worked on some point. Mine did not work from the beginning. There is plenty of space for my backup that's for sure.
Everytime I try to backup I get the "invalid file path" error after ca. 15%. At the download location it creates a file that is always between 850-900Kb.
I bought this unit from gadspot (GS1800), it seems to be AVTECH model 3 (no USB backup)
Thanks, Mark