Is backing up as simple as copying the device files (including hidden files)
Yes
or do I have to make a disk image of the device?
no
I'm afraid the 670 can only read a map that is no larger than 2GB. So you need a smaller img.
Then replace gmapprom.img, and remove .unl and .gma if your img is already unlocked.

Bluetooth SW version 0.00
Does BT work?