Getting Plex to access media on an external drive

Hello, I’m trying to set up Plex to use an external usb hard drive. Using Disks I set up the partition to mount. Attempting to add media directories to Plex shows the drive /mnt/tres but not the path to the folder /mnt/tres/Transmission. Transmission has no problem saving to the same folder. I tried setting permission with kuser, didn’t work. I don’t get it. Any suggestion on how to get Plex to see /mnt/tres/Transmission?

So, gave up on that. Rygel delivered to the smart tv, but not the roku.