And this disk is 5TB in size, right? Have you checked this out yet: http://www.microsoft.com/whdc/device/storage/GPT_FAQ.mspx
The disks are actually only 931GB in size. So it shows up as 3724GB which is (931 x 5) - 931, which must be the back up RAID drive.
I don't mind the RAID being partitioned if it's an advantage. The problem with my system is that it isn't allowing me to format the largest 1.6TB partition.