HMR said:
Uploaded maps always replaces all maps on the card.
HMR has provided an excellent explanation of the concepts, however, there is one tiny exception to the above statement that folks should be aware of. I only mention it to avoid possible confusion in the future.
If your GPSR stores uploaded maps on a data chip, CF card, or internal hard drive, and does
NOT come with preloaded maps, then HMR's statement is 100% correct. Whenever you upload a new set of map segments, the previous set is erased.
However, if you have one of the newer models of Garmin GPSRs that came preloaded with maps - in other words, one of the units that you simply take out of the box and start using, no fooling around with MapSource and the computer needed - then you have the possibility (but not the obligation) of having two different sets of maps loaded in it. One set will be the 'preload' that came with the GPSR, and the second set will be any map segments from a different cartographic product (e.g. Topo, BlueChart, home-made maps) that you have uploaded to the residual memory of the GPSR.
In this case, things work like this: The maps that are preloaded by Garmin when the GPSR is made reside in a protected area of memory, and, generally speaking, can only be over-written if you are updating those maps with a newer version, using a Garmin DVD. This includes, for example, updating CN Europe version 7 to version 8 by using the Garmin updater DVD and hooking your GPSR up to your computer.
The 'user maps' are not protected in the same way, and you can freely upload new map segments anytime you want. When you do this, you over-write the user map segments that were there before.
It's a minor point, and only applicable to GPSRs that come with the maps preloaded.
Michael