If the map you install is the original from your GPS then you can copy it to a new higher capacity SD card to run on. The Garmin unlock code is stored in the main memory with a file extension of *.unl. Connect your unit to your PC and Goto .system folder to see these files. It is wise to install Garmin maps into a SD card as any map corruption or error can result in an 'Unbootable' Nuvi.

To select the map to use in your Nuvi Goto Tools > System > Map >Info Check or Uncheck the maps you need. If you have more then one map for the region select only ONE at a time. Multiple map selections are know to give problem.