Hello. I downloaded a few maps from this wonderful resourceful site and loaded the IMG maps to an SD card. I followed the directions by getting the unique number of the SD to unlock the card with a .UNL file. The maps work on my older Garmin devices (a Nuvi 250W and a really old StreetPilot c330) but when I try the same SD card in my just purchased Nuvi 3790t it gives a "cannot authenticate maps" error. The same problem occurs on my friends Oregon 450t device. These two devices are very new models so I'm thinking Garmin has some kind of new unlock procedure? Please help!