Possible solutions based on the info you've given:
1. GarminExpress should have loaded GMA and unlock codes which authenticate and unlock the new map data for your LM entitled device. Perhaps something went wrong with the transfer and one of those codes is corrupt. Provided the device is still being recognized by your computer you can simply reload the LM mapping which should overcome your problem. In GE, click 'Map Options' and you'll be given the option to 'Reinstall Map' or if you have only a part of the LM entitlement presently loaded, to 'Change Map'.
2. If you've tried to load maps outside the LM entitlement then it may be that the problem is caused by that.
3. If the device is no longer being seen by the computer then a firmware cure likely will be required.
If you need more help please give as much information as possible.
Bookmarks