RAID is typically limited to 2TB for one array on a OS volume. This is a limitation to how the operating system addresses the data. The NVIDIA RAID on the 500, 600, and 700 series motherboards and the Intel RAID on the IHC10R (X58 motherboards) and P55/H57 have this limitation. This is not a feature that can be adjusted with a bios update.
Please Note: this is for a bootable operating system volume only. Storage raid for the X58, P55 and H57 can be up to 256TB would be the limit for the RAID.