hmmm, last chance, try:
correct your device.xlm to unit 3790 and older sw version, delete any GUPDATE.GCD from the unit ,put the new GUPDATE.GCD in the sd and restart it.
I had a problem with a corrupt device.xlm by an asian sw and it worked running the update from the sd in a Garmin folder.
Running it from .System, it never corrected the corrupt device.xlm