My motherboard is GA-MA790XT-UD4P, but I know others have had similar issues. Here's a quick guide I typed up to help anyone having trouble getting AHCI mode to work
0. Enable AHCI in the Windows registry (
http://www.ithinkdiff.com/how-to-enable-ahci-in-windows-7-rc-after-installation/)
1. Flash BIOS to latest using @BIOS. DO clear DMI DON'T clear CMOS
2. Connect boot drive to port 4 or 5
3. Set AMD SATA controller to AHCI mode
4. Set ports 4/5 to IDE mode
5. Set boot drive first in hard disk boot priority
6. Press F8 during bootup and boot into safe mode
7. Control Panel, System, Device Manager, IDE ATA/ATAPI controllers - if AMD AHCI/SATA appears, uninstall it and check box to delete driver (force Microsoft driver)
8. Reboot, F8, safe mode again (initialize Microsoft driver)
9. Reboot, enter BIOS
10. Set ports 4/5 to AHCI
11. Set boot drive to first in hard disk boot priority
12. Reboot, F8, safe mode (initialize MS AHCI driver on ports 4/5)
13. Reboot