Ah thanks. I didn't realise that you can't just swap drives in and out of the cache pool. I did a new config and reformatted all the non-array drives. Luckily I made a backup of my docker.img and my most-used VM. I'm transferring them back now. Fingers crossed.
I'm having issues mounting my drives. The option to mount is greyed out and no matter what I do I can't seem to find the solution. What am I doing wrong?