what i would suggest first are the basics:
uninstall all jaz/iomega software.
download/install the NEWEST iomega software/driver/extension whatever for the 9.2 g4
check all connections, plugged in securely/clean
try mounting other iomega products if you have, like an old zip drive and see if that works
plug in your friend's jaz and see if it works. use your jaz cartridges. other wise, get a new jaz drive....having it repaired is not worth it