/c/ just GONE after reboot??

Hi guys!

So, I have a ReadyNas Pro 6 which was mostly populated with 2TB Samsung Spinpoint drives, which I have gradually been replacing with 4TB Seagate NAS Drives.
Yesterday I replaced another one of the Spinpoints with a WD Red 4TB drive. It checked the disk as usual but instead of then expanding the volume, it just flagged the drive as a Spare.
I rebooted the ReadyNAS to see what would happen and from then on I got email alerts saying "Volume scan failed to run properly" and "Missing shares detected".
Indeed, it would appear that, all of a sudden there is no volume on Frontview.

I SSHed into the box and, sure enough, /c/ doesn't seem to be there any more, for some reason.

Is there any way anyone can suggest how I could retrieve my data? I have 10TB of stuff I would REALLY rather get back, as you can imagine. I can only assume the data is still physically in teh drives, as there has been no Factory Reset and I certainly didn't delete anything...

Any help with this would be greatly appreciated...