It appears you have your disk auto-mounted under /media. This is generally a bad idea because of how Ubuntu/Gnome handle ownership/permissions of auto-mounted disks. Specifically, no matter how you set permissions, no user other than yourself will be able to access the data contained on such a mounted disk. This includes the user plex, under which Plex Media Server runs.
This guide shows how to set up manual mounting for the disk(s) and assign correct permissions so that Plex can access them:
If you want to try something else, which I’m not sure will be supported, you can try modifying the ACLs for the auto-mount home for your user to allow Plex access. To do so, run the following command:
sudo setfacl -m u:plex:rx /media/dad/
This will allow user plex read and execute permissions to the dad directory, under which your disk is being mounted.
Just to be clear, the forum post I linked above is the recommended way of giving Plex access to your externally mounted disks. The other directions I posted are just something you can test. I myself have had success this way.
