What you could have done is RAID 5 the whole array, set up several partitions. OS(100GB) then separate the rest out in to logical partitions. Then make those a volume set inside of disk manager and pull them into one large volume within windows. I have done this frequently to get around the 2TB limit in MBR partitions. I am not worried about a volume set failing since RAID 5 is underneath. Think of them as stripped disk with no parity...and no striping.
Interesting way to do it, I thought of something like that as well but knew there was a better more "best practice" way to do it, although this was similar to my initial fix. On another note, I updated the BIOS and there is some reference to UEFI now.... yet I don't think it supports it as there is no reference in the printed or electronic manual that I saw or on the features for that board. I'll take some screenshots next time around to show what I'm talking about for the new UEFI reference.
Since its a server motherboard for a SMB I'm using the on-board controller, which consists of 6 SATA ports, 2 x 3gb/s ports and 4 x 6gb/s ports. So the RAID 5 is on the 6gb/s ports and the new RAID 1 on the remaining 2. My only issue was not having another SATA port for an optical drive lol. Not a big deal as I simple made a flash drive installer of server. Fun fact, if you put your optical drive on a PCI SATA controller, windows may or may not recognize your RAID via an on-board controller even after you provide it the proper AHCI/RAID drivers that it recognizes on an external flash drive or the optical drive. Went around for a little over an hour with that issue last night until I just made a USB bootable Windows installer and then magically it wasn't an issue