Is there enough space on a European Fenix 5X to upload US maps ? Mine says no...

For sure there is not enough space, because when EU topo maps had an update, even topo maps were not able to update due to size, we had to copy out old map, and from new map to select which part you wanted.

In your case you need to connect your watch to PC and check via Basecamp what is the name of EU maps you do not want to have and cut the respective file out of your watch, you should store it somewhere, when you will want it back then you do vice versa. The name of file will be xxx.img and will be very big (3 G )

One more point: When I had my watch new, I also bought maps, I would not do it again. The reason is, that there is plenty of Free maps for  Garmin devices, for my country the free one is better than Garmin one. Not sure about USA though, I would be surprised if there are no open topo maps.

Oh thank you so much, I'll check that out with my Mac. 

Where should I store the EU map?! I have no idea about that ;-)

somewhere on disk of your Mac, where you find it when you need that. :)

Sorry, me again. When you say "cut the respective file out of your watch" you mean do it from the watch or from Basecamp??? I can't seem to be able to do it anyhow ;-(

In basecamp you can see which map is what filename. And then you go to Mac file explorer to the USB disc Garmin as watch look connected to computer and move the respective file name out of Garmin disc to Mac. The name will be something like gmapprom1.img or so and will be quite big. You can also check the space before and after this operation.

welcome, let me know if you managed or need some more help.