Setting up samba shares on linux is not fun. Especially if you're trying to share content on a seperate drive.
However, I figured it out. You'll want to bind the external drive to a path under /var/
Here's what I did, to test (not necessary):
mount --bind /source/ /var/samba/folder/
If you're happy, modify your nano /etc/fstab:
/var/run/media/user/sysdrives/drive2/Torrenting/distros /var/samba/distropub none bind,ro 0 0
The ",ro" is for read only. You can change that if you'd like. Make sure to chmod the /var/samba path to get things working 100%.
There'll be something saying systemd isn't updated. Just follow what it says to update systemd. It'll prevent errors at boot.
Now, modify /etc/samba/smb.conf and make a new sambashare to something new.
Here's an example from mine:
[Distros] comment = Distros browsable = yes guest ok = no read only = yes create mask = 755 path = /var/samba/Distros valid users = user

Then, mount everything in /etc/fstab and restart samba
sudo mount -a
systemctl restart smb