The reason I ask is the FIRST COMMENTS I FOUND about this wrote the motherboard bios must be current, you did that and it's still dead.
Just completed a new build with a Gigabyte GA-990FXA-UD7 and a AMD FX6100 (6 cores). After OS install I ran task manager and it only shows 3 cores, same for CPU-Z, just 3 cores. I don't get it! I had this CPU installed previously on a GA-990FXA-UD3 and I could see all 6 cores. Only change I have made is to update BIOS from F5 to F7 (the latest). Using Win7 64bit if that helps.