This is the info on my hardware as it was when the issue occured:
iMac
27 inch mid 2011
3.4 GHz Intel Core i7
16 GB 1333 Mhz DDR3 RAM
HD Enclosue
Mercury Rack Pro http://eshop.macsales.com/item/OWC/MRP1UF8U3EP/
Connected to iMac via Firewire 800 (DAS, no daisy chaining)
Hardware RAID (Configured to RAID 5)
Drives
Seagate 4TB FW: CC52 (x4)
No other drives were attached to the computer at the time. No software from WD or anyone else being used as a RAID controller. Hardware RAID only (so yes it could be an issue with the firmware in the enclosure)
Here's what happened: After installing 10.9 on my iMac the RAID (which I had named 12TB RAID) showed up with 2 partitions, one an EFI partition and the other named as MyBook. It was displaying the entire drive with it's 12TB capacity as blank. The fact that the drive mounted with 2 partitions and one of the partitions was named Mybook is strange, as that is Western Digital's designation and as I say, I have no WD software on my machine.
I since plugged the drive into a Macbook Pro running 10.8.5 where the drive showed up on the desktop as just the MyBook partition, again 12TB of free space. The EFI partition does not show up as mounted on the desktop. Running a quick scan on the drive with Data Recovery Software showed there is at least some data on the drive somewhere. Rather than go through the whole scan and recover process I cancelled the scan and ejected the drive.
To clarify, there is no 3rd party RAID controller software on either my iMac or my Macbook Pro. No Western Digital software at all. No OWC software. I'm using hardware RAID only. I've contacted both Apple and OWC about this and both are investigating the issue.
I do use Western Digital drives, but not in this case. I've never installed any WD software on my machine but I do wonder if somewhere one of my WD drives had managed to discretely install some kind of device driver or something on my machine that migh have played a hand in what happened. If anyone knows how to find out if that is possible and if so if it in fact is part of the problem, that would be great.
So far, the response I've had from Apple is that it may be possible that Mavericks is somehow showing the drive as empty, when in fact the data is all still there, rather than the fact that Mavericks formatted the drive. The possibility being that some instruction has been written to the drive to display it as blank. Though he couldn't give me much more precise info on that as he said the engineering team are still investigating.
I'm pushing at the limits of my computer knowledge here, but I vaguely understand that an EFI partition would include some of this sort of info on how the drive shows up when attached to a computer, is that correct? If so, might the fact that Mavericks has done something to the drive to alter the way it shows up (i.e. as a blank drive) explain why the EFI partition initially showed up alongside the Mybook partition, and why the drive now shows as blank when plugged into a computer that does not have Mavericks installed?
I was advised not to attempt any data recovery yet as there may be a solution whereby it would be possible to set the drive to show the data properly. This was presented as the best case scenario. Worst case scenario being that the drive has in fact been somehow formatted by Mavericks and rebuilding from back ups and carrying out data recovery is my only option.