Cyberjock, I have already pointed out I've read advice on the use of RDM and looking to migrate to a more recommended solution, however I only happened upon the thread earlier this week. I built the box a while back before this info was readily available.
I've managed to revert and data is all good and I keep an offline copy anyway
A big thanks to the Freenas team for an awesome system, ultra impressive work.
If any further troubleshooting would help others let me know.