Loading supplemental maps (such as the free topos from GPS File Depot) to Garmin units often confuses people. If you use MapSource or MapInstall to load a map, it works fine. But load another map, and the first one you sent is overwritten. Here’s how you get around this…
Older Garmin units
For older models, select the portions of the map you want to send to the unit, then use the drop down Map Selection box and choose another map, as shown below. Select the portions of this map that you wish to transfer to the device. Rinse and repeat. Once you’re finished, transfer the maps all at once. Unfortunately, if you decide the following week that you need another map, you’ll have to repeat the process again.
Newer Garmin models
UPDATE: You no longer have to do this.
On newer models with updated firmware (Colorado, Dakota, Oregon, GPSMAP 62/78 and nuvi 1xxx and higher) you can change the map file names. The advantage here is that you can give the map file a more memorable name and make it easier to load additional maps should the need arise.
Choose a map and transfer a set of map tiles to the device. Now use your file explorer and go into the Garmin drive, then into the Garmin folder, and find the gmapsupp.img file, as shown below. That’s the supplemental map you just transferred.
Now rename the gmapsupp.img file to something more memorable, retaining the .img extension. An example would be Utah24Ktopo.img. Now you can send another batch of maps to the device, rename them, and repeat the process to your heart’s content (or the limits of the unit’s or microSD card’s memory!).
Important: If you have any qualms about your abilities to do this, distinguish between similar file names, etc. , back up your device first. Just copy the entire Garmin drive contents over to your computer. It takes awhile, but I actually recommend you do this with any new unit before doing anything else with it.