The permisions on the drive are only open for the owner. I am not familiar with that software but unless it runs with root permisions it will get denied.
Change the permisions to 770 then add yourself to the group the software is.
You would need to use sudo / su to process the commands. I have sudo in the examples below.
sudo chmod -R 770 /path/to/drive
sudo usermod -Ga plex your_user_name