Check permissions of the external drive, and set owner to jellyfin group. Give rw permissions to that group/drive. I went through similar with my external drives. It was a permissions issue.

Yeah I reread the part where you said you installed Linux lol. On top of checking permissions also check user ownership of the media. I believe a normal jellyfin install uses your user rather than it's own system user

Installing Linux and running a service is hard and if you're only going to be running Jellyfin then most people would say that's the best way to go (and probably much much harder than just doing a docker-compose up command which is pretty much what I did). Well done on getting it going. I've been running it a few weeks at home, yesterday I got to test it out remotely, I was stunned that even with my rubbish home internet, it ran flawlessly!