Have you found the solution? I have exactly the same situation.
I have Raspberry Pi which is running Libreelec/Kodi. I have hard drives with movies connected to the Raspberry and accessible in the local Kodi GUI, and I also have them shared with my PC through Samba, so I can easily manage it.
Now I want to add one more drive with personal files, to share only with my PC over SMB, but not show up in Kodi.
I want this new HDD mounted all the time and automatically after reboot. Is there a solution to this?