That's cool, very useful to know the naming methodology and the function of the various IMG files.
I have just tried renaming the gmapsup2 per your instructions but the Zumo still does not "see" this second mapset. I know this mapset works because it will pick it up if i call it gmapsupp.img

As to the SD card question, i was simply asking because i've read here that if you generate the key for the SD card than you can plug the SD card into other compatible Garmins, that's all.