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I can only see 4 maps (3 CurseView and 1 SkiView); there's another one that loads just when start the application, but disappears right away (gmapbmap)

I can only see 4 maps (3 CurseView and 1 SkiView); there's another one that loads just when Basecamp starts, but disappears right away  (gmapbmap)

gmapbmap is the Garmin base map and won't show in the map list.

this is the basecamp screen

That's what I'd expect to see.

Some days ago I can see other maps that also are stored on the device.

The app also does not allow creating a backup.

How can I fix it?

Sorry I'm now unclear on your situation. What other maps, as you only mentioned the 4 that show earlier, plus the base map which won't show as I've explained. You can also create a backup, via the menus in the toolbar.

In the forerunner there are another map (OSM ) that some days before are showed in Gramin Express,

I can use these map on Forerunner

I think Garmin Express must show the other map and data from internal storage.

Not all OSM maps are visible in BaseCamp. This is an OSM and not a BaseCamp issue. You can set them to be visible in BaseCamp using JaVaWa's Device Manager program although I don't know if it will work on your device

www.javawa.nl/jdm_en.html