Hit 8Tb limit - how to expand to 18T physical?

Thanks for the quick help StephenB, much obliged.

I've been busy since yesterday. I hot-swapped drive 3 from 2Tb to 3Tb, so now I have 3x3Tb in bays 1-3 and 3x2Tb in bays 4-6. Drive 3 took about 6 hours to do the RAID sync last night, and is actually now restriping (currently 74%) and has been for the last 9 hours, with a predicted 11 hours to completion. That seems very slow - is it normal?

Now when you say 15Tb, I assume you mean with 6x3Tb installed, is that right? If so, my sequence would be:
1 Upgrade drive 4 to 3Tb, making 4x3Tb + 2x2Tb (but still limited to 8Tb volume)
2 Back up the data (plus the OS and config) over the LAN
3 Factory reset and firmware update
4 Restore the OS and config
5 Restore the data
6 Upgrade the final two drives from 2Tb to 3Tb

Does that sound good?

I'm still trying to find a way to back up. I have installed rsync on my Mac Pro but until I can make the network authentication work, I'm a bit stuck, unless I want to go into the command shell and use tar myself (which I could do, but I'm nervous about screwing it up). A friend has suggested installing a VM on the Mac, and installing Linux on that so that NFS becomes easier (OS X makes it hard I think). I'm not sure I want to do that much stuff to my Mac Pro, as it's supposed to be a carefully-controlled workstation (I write music on it).

I'm the only user at the moment, but there are a couple of different file owners, because I am using the WD TV Live SMP and I found that it didn't work correctly unless I changed the media files to be owned by a special "nas" user. I can't remember exactly why now! But I'm not too worried about the permissions really, since I can always list them all before I start, and then change them back after the restore if I need to.