When moving all my files onto an external drive Plex was able to display the drive but was unable to read the content of the folder.

Changing group ownership and permissions did not solve this problem.
The drive was formatted with NTFS before being installing it in a housing and connected to the Linux machine.

The problem is caused by the default mounting parameters when first connecting the drive. In order to fix it you will need to manually setup the drive by editing the fstab file. Use this command

sudo blkid

This will get you the drive UUID of the drive that you need. Look at the labels until one of them looks familiar. You can also use the built in disk utility to get this number. System > Disks > “Gear Icon” > Edit Mount Options but it’s harder than just doing it in the terminal.

Create a folder to use as the path for your drive. I used /media/Videos since that’s what I would be storing.

sudo mkdir /media/Videos

Open the fstab file and add this line to the end using the UUID that you just found.

sudo gedit /etc/fstab
UUID=E12345A1234C1A12345 /media/Videos ntfs-3g defaults,permissions,auto 0 1

It’s easiest just to restart your computer to get these changes applied. You could also unmount the drive and then run the mount command which will pick up the new settings from the fstab file. If you have trouble with this just reboot

sudo mount /media/Videos

Without doing the next couple of steps I didn’t consistently have problems but I did have problems with permissions and file transfer being denied

cd /etc/samba
sudo gedit smb.conf

Scroll to the bottom and add into the section that was just created with the name of your share [videos] in this case

force user = yourUserName

And then restart Samba

sudo restart smbd

This other post helped me get up and running if you want another take on the same process.
http://travelinlibrarian.info/2013/05/how-to-share-an-external-usb-hard-drive-from-ubuntu-to-a-windows-network/