You need to load the AMD RAID Driver at the beginning of the Windows 7 installation.
Put the two drives on the first two AMD SATA Ports.
Press the Control & letter F keys on bootup to enter The RAID Configuration Utility.
The motherboard manual explains how to create the RAID in the Appendix of the manual.
Download the AMD RAID Drivers Package
http://support.amd.com/us/gpudownload/windows/Pages/raid_windows.aspx#2Insert a USB Flash Drive in your PC.
Click on the file you downloaded. It will ask where you want it installed. Select to install it to your USB Flash Drive.
When the installer finishes extracting the files and folders, the Catalyst Install Manager pops-up. Just CANCEL that.
Look at the folders you've extracted and you will find the SB 850 folder (SB 850 and SB 950 are the same hardware).
The W7 64 is the folder to direct the Windows installer to in the beginning of W7 DVD installation. Select LOAD Drivers when the Installation starts.
Select the W7 64 folder and it will ask you if you want to load the AMD AHCI COMPATIBLE RAID DRIVER. Select Yes.
After Windows installs, you can run the Catalyst Install Manager - the program you previously cancelled and the AMD Chipset Drivers.