I've been getting success on the 205, but the procedure totally failed on the 350NA. The only procedure that worked on BOTH units is unlocking the official file with the more modern version of gimgunlock and putting as gmapprom on the internal memory. I suspect the over 2GB file issues with some units, but the trick I found is you can put a 1GB dummy file on the 55LM unit and then they offer you the option to download a trimmed map via Garmin Express for your region, which you can then unlock and have available for the sub-2GB units. This way you don't have to worry about using an SD card.

But what's this I hear about bad maps bricking the unit? In my experience the maps just don't get detected or don't work right if they're bad.