The reason you can’t see the disk in your Linux installation is that it is being auto-mounted and has access control lists managed by udisks2; no amount of Unix permissions changes are going to give user plex access. The recommended solution is to create a dedicated mount directory and mount the disk to it, as described in this article:
If you want something easier to try first, you can modify the ACLs so that user plex has access to the automatic mount folder for your user. To do this, run the following (change the directory path as necessary):
sudo setfacl -m u:plex:rx /media/user/
You can change “rx” to “rwx” if you’d like user plex to also have write access to the disk.
