FAT32 doesn’t support unix file permission, so when you mount the disk linux has to assign a default ownership which usually is to root. And this is the issue you are facing.

You confused the disk permission with the filesystem permission. The udev rule you wrote gives you permission to write the disk (in other words, you can format it or rewrite the whole content) but doesn’t give you permission on the files stored inside because they are on a higher abstraction level.

If you use this computer in interactive mode (in other words if you usually sit in front of it and plug the disk on demand) my suggestion is to remove that line in /etc/fstab and let the ubuntu desktop environment mounting the external hard drive for the current logged in user.

If you use this computer as a server with the USB disk always connected (likely since you mention Jellyfin) you need to modify the fstab line to specify which user should get permission on the files written on the disk.

You can see the full list of options at https://www.kernel.org/doc/Documentation/filesystems/vfat.txt

You either want uid=Mongostein (assuming that’s your username on your computer too) to assign to yourself the ownership of all the files, or umask=000 to give everyone all the permissions to the files and directories while ownership will remain to root. You should prefer the second option if jellifin runs as a different user, while the first one is better if there are other users on your computer which shouldn’t access your external disk.

To summarize, the line in /etc/fstab should be one of these two.

LABEL=drivename /mnt/drivename/ auto rw,user,exec,nofail,x-gvfs-show,dev,auto,umask=000 0 0

LABEL=drivename /mnt/drivename/ auto rw,user,exec,nofail,x-gvfs-show,dev,auto,uid=Mongostein 0 0