That “+” at the end of your permissions in the output above means that ACLs are being used for managing access to these files/folders. This is due to the drive being mounted into /media/USER by the udisks2 utility. ACLs trump normal Unix file permissions; no amount of chmod’ing or chown’ing will allow anyone other than your user to view those files and directories.
It is generally recommended that drives being used for Plex be mounted with dedicated mount points set up in /etc/fstab. There is fairly extensive documentation, with examples, in this how-to from these forums:
This is not recommended, but you can also modify the ACLs for that mount to allow access to another user or users using the setfacl utility. Something like:
sudo setfacl -m u:plex:rx /media/USER
The reason this isn’t recommended is that there are potential timing issues for the Plex service and when the disks are mounted by udisks2 which could cause media to be unavailable at the time Plex starts. Do yourself a favor and create static mounts now.
