It is relatively simple.
The maps that can be installed by the users on the Garmin devices come in the
.img format*.
When you normally download or purchase the maps you get given the option to download the .gmap file, to be used with BaseCamp, and/or the .img file, that is the same map, but in a format that the Garmin GPS device can read.
Now:
older devices can only read one .img file at a time. And generally this always have to be named the same, hence the famous "gmapsupp.img".
Older devices, like my 60csx, will look for the gmspsupp.img file for the maps (location:
/GARMIN/). Ignore any other .img files with different names. If nothing is found, it will use the embedded base maps.
You can combine multiple maps in a single gmaspsupp.img file, but (for me) it is a faff.
As written above, as I constantly use two sets of maps on my 60: Great Britain routable open street maps, and some non routable openstreet maps with greater detail for greenlaning.
You can keep multiple maps on a memory card and rename them accordingly, but I find it easier to have 2 memory cards with the two different files, swap them when needed.
I keep the spare card in the battery compartment. A little duct tape as padding to keep it wedged in place between the card compartment and the batteries.
On newer devices, like my XT, you can have multiple .img map files (location:
/Map), with whatever names you want. The unit turns on, scans memory for files and, when found, these will be available as extra maps you can activate/deactivate manually.
On the XT, I have the Garmin maps, plus non routable OSM for greenlaning (gbopen in the screenshot) as above and, for this example, I also loaded another set of maps: TalkyToaster's.
You can see both "custom" maps in the screenshot within the XT's memory and then available in the MyMaps menu.
(You cannot see the original Garmin maps in the XT filesystem. I guess those are hidden to prevent people from copying them around.)
And here you can then activate them from the settings:
I honestly don't remember when the cutoff between "old" and "new" device is.
I was using multiple maps on my Zumo 396 as well, in the same exact way as I'm doing with my XT. I think that was feasible for the 660 too.
Fair to say that most GSMap devices up to the 64 might only read one map at a time as described above, but not 100% sure.
*not to be confused with MacOs X .img volume image file format. Not the same thing.
