Disk Utility would see the drive and the partition (greyed out). It couldn't verify or repair the partition. fsck was having none of it either. It took all night for the Raid device to verify itself, and it claimed all the bytes were in the correct place and there were no disk errors.

My strategy of having a Raid6 device with 6 drives (double stripe and parity) for security was looking rather weak.

After a bit of googling, foruming and $112 dollars I downloaded Disk Warrior. 10 minutes later and all is good.