ddabcd277, aekhong
Added some info about supported devices. Will add a manual a bit later.
I don't have any handheld devices, so I need someone to test it. The tool should be safe, because it doesn't add any functionality into firmware, it's just an old plain mass storage mode.
Nuvi 30,40,50 should be ok. The only problem with that devices is digitally signed GCD (not encrypted, just signed), but we just don't use it here.