Note: I know nothing about Plex. The only thing I keep seeing in posts is that it runs as the user plex but that is my extent of knowledge.
In my test box when I ( user = tester ) attach a USB HDD it mounts to /media/tester/$UUID - accessible to tester but no one else.
One way to have universal access to any USB media is to use bindfs to create a "view" that allows it.
[1] Install bindfs:
sudo apt install bindfs
[2] Create a mount point for this "view":
sudo mkdir /MyMedia
[3] Temporarily remount /media/tester to /MyMedia using bindfs:
sudo bindfs /media/tester /MyMedia -o force-user=tester,force-group=tester,perms=666:+X
If I attach a USB HDD to the system it will now appear to be mounted twice:
** Once where Linux wants it to be mounted:
$ ls -al /media/tester
...
drwxr-xr-x 3 tester tester 4096 Dec 31
1969 03E9-7C8D
** And again where bindfs mounts the "view" with a new set of permissions:
$ ls -al /MyMedia
...
drwxrwxrwx 3 tester tester 4096 Dec 31 1969
03E9-7C8D
When I create the samba share I point it to /MyMedia
To undo the bindfs mount:
sudo umount /MyMedia
If it does what you want it to do you can have this bindfs "view" created at every boot by adding a line at the end of /etc/fstab - with a change of syntax:
/media/tester /MyMedia fuse.bindfs force-user=tester,force-group=tester,perms=666:+X,nonempty 0 0
Then unmount it if you still have it mounted:
sudo umount /MyMedia
Then make systemd happy:
sudo systemctl daemon-reload
Then mount it:
sudo mount /MyMedia
It should now mount that "view" at every boot.
Like I said at the top of this post I know nothing of Plex but it it wants all files to be owned by the user "plex" replace "force-user=tester" with "force-user=plex" in the bindfs mounts.
