Older Garmin devices (such as the GPSMap 60csx) only recognized one map file that was named gmapsupp.img. Since Mapsource is old, it always names the map file gmapsupp.img. Garmin had an older system of encapsulating multiple maps into a single gmapsupp.img file and while this is still supposed to work with never devices, I have had problems with it.
So the best solution for a newer device like yours is to put each map in a separate file. The work-around for this is to send your first map to the GPS, then manually change the file name to something else. So, for example, change gmapsupp.img to topo1.img. Now go back to mapsource and send another map and change its name to topo2.img. Just continue this process until all your maps have been sent.
You can use any filenames you want, but the point is that you can't leave the original gmapsupp.img file because when you send another map it will simply replace it with a new gmapsupp.img file.
Now MapInstall is more modern software and if it detects you are using a newer device, it will automatically create separate .img files with different names. But unfortunately, the newest version seems to be broken.
