Hi Roy - I too have migrated to off Windows Manjaro about 2 years now. I longer a dual boot but I run Windows in a Virtual machine..I am not super expert in Linux yet, but have treaded similar path as you.
I think you may have answered your own question. If you are using Disks as per you screen shot, then I suggest hitting the cogs wheels and edit mount details, to create different mount points for your different devices (UUIDs) and then make sure they are mounted on boot. MC is relying on the linux filesystem underneath, so if you mount different device to the same place or you let linux assign it (run/media/ etc/) the results can very and definitely overlapping different removable devices being mounted at the same file path is possible - which will cause problems if you are trying to treat them as different.