I downloaded the AppImage file from the GitHub Repo and I did check that my USB Drive is connected which I can access it through my File Manager as well as Gnome’s Disk Utility.
Oddly, when I tried to set an directory for Movies and TV-Shows in MediaElch, it acts like I hadn’t plug my USB drive.
Usually this is a problem of permissions on the mounted-folder. Even with the jellyfin server you need to change the permissions to get the server to see the mounted folder. I expect it’s similar with kodi.