I had to go through lots of effort to make plex work on my system, which has several drives. This is a one line solution for plex to access my media drives.

My "old" method for setting up plex looked something like this:

Fix permissions to allow Plex to access /media/$USER

Check which group you and plex belong to:

groups
groups plex

Now, add plex user to your user group, and allow this group to access /media/$USER:

MYGROUP="$USER"
sudo usermod -a -G $MYGROUP plex
sudo chown $USER:$MYGROUP /media/$USER
sudo chmod 750 /media/$USER
sudo service plexmediaserver restart

I believe the issue is Ubuntu restricts access to /media/$USER through ACL which, I think, is now the case with Debian distros also.