Sorry, just to clarify: I wasn't formatting a drive at the time that I lost the data. In general the process I have been using is: format a drive so it is fresh, add it to the pool (without duplication or scheduled balancing) and then copy data into the pool from another drive, freeing that drive up in the process. I would then format that drive and add it to the pool etc.
I had just added the third drive to the pool and was copying data from a fourth drive (not in the pool) into the pool and that is when I appear to have lost all of the data. The Folders that I had inside the PoolPart folders are now reporting Owner as "unable to display current owner". So something strange has completely ripped the pool apart here. I'm still struggling to put together the pieces. Windows may have also downloaded/installed some Windows Updates during the night (great Windows 10 feature that I can't seem to prevent) but somewhere along the way the combination of that, the copy process and the pool it has managed to wipe 6.5TB of data. I really want to use this product but I need find out what happened here and make sure it doesn't happen again.
Out of interest does anyone have any experience of some good free data recovery tools for files larger than 4GB? I have used Recuva to scan the drives but it reports all files over 4GB as unrecoverable. This seems to be a limitation with the way that the NTFS deletes files over 4GB but there must surely be a tool out there that can recover them.