Thread: Nuvi 3790T Demo Mode

According to the following link, you need to remove 0:/Garmin/demo/demo.srf to disable demo mode.
[Only registered and activated users can see links. ]
If you can access your device's internal flash drive, then you're ok. Otherwise you can use GarminCure3 to access it. Or you can use update.txt script, as suggested in the topic above.

UPD:
Also, if you find GarminCure3 too complicated, you can try this way of entering into the mass storage mode: [Only registered and activated users can see links. ]
It's really quick and it definitely works on your device model.