I already tried:
1. sudo nano /etc/default/plexmediaserver --> PLEX_MEDIA_SERVER_USER = USERNAME
2. "sudo chown -R USER:plex MEDIAFOLDER" in combination with "sudo chmod -R 770 MEDIAFOLDER"
Before I tried the second option i completely wiped my Computer (I'll purely use it for Plex).
I have no Idea why but it's simply not working. When I want to add my Mediafolder it's not opening the external HDD. I can see it but when I click on it nothing happens. When I want to change the Mountpoint to the home dir my OS is only starting in TTYL and i need to delete the line in /etc/fstab which i created for the Mountpoint.
Any help here maybe? I'm out of ideas.