OS is on Sata2_6 is GIGABYTE SATA2 chip which is a secondary storage device. The main SB850 is not being used for the OS. I recommend not using the GIGABYTE SATA2 chip.
You need to use one of the latest versions of the RAID drivers for drives over 2TB.
I would advise you to enable RAID for the SATA3_0/1/2/3 connectors, set OnChip SATA Type to RAID.
Connect the drive for the OS to SATA3_0.
Download the latest AMD chipset drivers.
http://support.amd.com/en-us/download/chipset?os=Windows+8.1+-+64Extract the file to a USB 2.0 thumb drive by clicking on it. Cancel the catalyst manager when it pops up.
Explore the folders till you find SB8xx RAID Drivers that are in the SB8XX RAID folder. You want to direct the Windows 7 installer to load the FOLDER marked for your OS->W7-64bit (similar name). You have an opportunity to load the driver (FOLDER for your OS) at the beginning of the Windows 7 installation.
After installing Windows 7 you can rerun the AMD chipset drivers and let the catalyst install manager do its thing.
Now connect the 3TB drives to SATA3_1 and SATA3_2.
Create the RAID array. In Disk Management you should be able to do a GPT format on the RAID array.