Ok! I seem to have got misen into a bit of a tangle........................
Taking Emegra's advice, I deleted the 2 partitions on sdb and installed Zorin on that drive, using the whole disk. The installation seems to have gone well - well, it's on and looks to be working ok as it recognises the sda drive and it's partitions.
However.......... when I boot, the Grub screen has Zorin (listed as Ubuntu, btw) in top spot, with Mint 13 several places down. Also, there are several other versions of Mint/Ubuntu listed below these (prior trial installations) + the Memtest entries etc.
What I need to do is edit the Grub menu to
1 put Mint 13 at the top as default (and rename it)
2 put Zorin 2nd (and rename it)
3 put Mint 12 3rd (as a back-up O/S in case 13 goes awol)
4 clean up the rest of the entries
I've installed Grub Customizer but I'm totally confused as to using it - plus when I click the general settings tab it just closes??
Hitting the 'e' edit key at the Grub screen brings up another screen that I have no idea about! I've read a few 'googles' re: editing Grub but I don't want to screw things up by deleting the wrong things!
Also, my main drive (sda) has 9 partitions on it which makes it difficult to know what is where!
I think it's down to me possibly choosing 'something else' when I first installed Ubuntu - but it was 18 months ago! I'd like to reinstall Mint from scratch but will start another thread for that if it's appropriate.
Hope all this is not too confusing?