Yes, i second the above opinion. Unfortunately that sounds like it's probably damage to the flash memory anyway.

If it's running usual Garmin proprietary OS and firmware and you can also determine which is its nonvol region (41; 154, or ??), you could dump a copy of its data if that's even possible, and if it is possible then erase it to clear any possibly badly corrupt data. I doubt those devices can even dump regions successfully though, even if they run traditional garmin os/fw, unless it's got a hidden RWFS partition in the flash to copy to. You could look at it's firmware and if it IS usual garmin type, check if it has rrgn and ergn commands written in it to dump and erase the nonvol region.... but if not it's probably toast whether it's physical damage or corruption, sorry.

Garmin's exchange program might be your best bet - they don't repair and return, only replace in kind.