At the moment, this sd card can't be used. Hopefully our specialists find a way to change this status.
I once saw an unused card (don't have access to it anymore). There were *.unl files on it.
Probably when the card is used for the first time in a car, the maps are unlocked in combination with the FIN of this car.
As lolypop000 wrote, changings of the garmindevice.xml will probably have no effect, it's garmin style to create a new file during the start-up procedure.
So the maps are only useable in the car with the correct FIN (and maybe other codes of identification), until a way is found to unlock the maps generally (like it is possible on garmin PDAs)