The method to back up the maps is to
1. send them to the GPS from Basecamp if they're not already in the GPS
2. open up the 'Garmin' folder of the Internal Storage of the GPS (please be careful!)
3. locate and double click on the 'Birdseye folder'
4. Copy and paste the .jnx files from the folder in to a folder in your PC/external storage device
Please note the maps can't be used outside Basecamp/sent to anyone as they are locked to the serial number of the GPS.
We urge caution when performing this operation and please be careful when you are copying and pasting the files from the Birdseye folder.
Loss of data/deletion of files/folders on your GPS is done at your own risk and we do not accept liability for any such loss.
This is covered in the On Line Resource, Basecamp Top Tips. While it is for a Windows PC, the procedure is identical for a Mac.