First, sorry for not providing screenshots with exact settings, but I am not at home now, but I will try my best
You have to look into reForis packages and allow Samba there: Package management → Packages - NAS → Samba
Hit save and wait while Samba is installed.
Then you have to go to LuCi (Advanced administration) and there look into System → Mount points, where you should see your connected disk - e.g. something like /dev/sda. If not, you have to create (add) mount point for that disk.
After that go to Services → Network Shares and set there your preferred share directories.
Don´t forget (!!!) to enable Samba service autostart in System → Startup. Hit restart button for restarting Samba service too.
Now everything should work. To access shared folder you have to visit
smb://your_router_ip/Shared_folder_name from your Mac or Linux PC.
To be sure everything will be working, you can try to reboot your router and check if your shared folder is accessible after reboot.
If something is not clear, let me know, I will add some screenshots or config files with exact settings.