You don't need cure firmware., you just need original US gupdate.gcd saved to .RGN file by RGN_Tool to flash your US 3790 back to use original firmware.
Hard reset won't help.
Download link ::2). RGN_Tool ( [Only registered and activated users can see links. ] )Code:Please Login or Register to see the links
3). .RGN file I said above.
4). Access to pre-boot mode for flashing nuvi back