Jimbob,
I have a Garmin 60CSx, with the basemap it came loaded with. The 100K map came off Topo 2008, which is a separate disc that I opened in MapSource. From what I can tell, the basemap is loaded in internal memory, and the stuff I've transferred has gone to the mSD card. Of course, now I have to tinker and get my system (Windows 7 with multiple drives) to recognize the card! I can read it through a card reader, just not through the unit? And I should have checked that the factory installed card was the 64mb one listed, and not the 1GB surprise upgrade...would have not picked up a 2 GB one at the store! But, such is life.
Thanks for the scoops. Good to know every transfer of maps will overwrite what's presently on the card! Not on the card were several routes and waypoints that I also transferred. Do all routes get stored on the internal memory?