However this is something that does need to be fixed. So I bought some more hard drives. I'm using Western Digital Greens from ebay with the head park adjusted with WDIDLE3 to extend their life. I know these are not ideal drives for use in a NAS, but they are cheap and the RAID 6 configuration means that I have time for a new drive to arrive if one dies.
Once the RAID card has expanded the drive I think I will need to expand the filesystem so I will do a writeup on that because I will need to remember as well.