@Barry. Welcome to the forum
If you had Mint as dual boot before, I assume you know about linux partitions and filesystems. /, /home and swap. Maybe you forgot to set you Mount Point to / when setting these up. That caught me out once.
PS. I have corrected "Duel" to "Dual" in thread title, which I assume was a typo.
Also most of us are on Z15.3 or Z16Beta. Do you have a reason for choosing Z12?