Gpu not running at proper performance

I spent nearly 1000$ on my new 2013 build and made sure of every piece being compatible and optimized for the pc, days after my new build i noticed my new graphics card was getting way low fps, and unable to run anti aliasing. the one accidental issue is that i installed my os (windows 7 64) and all the drivers for the components on my 350gb seagate external hard drive.

computer parts
GPU- ASUS HD7850-DC2-2GD5-V2 Radeon HD 7850 2GB 256-bit GDDR5 PCI Express 3.0 x16
mobo- GIGABYTE GA-X79-UP4 LGA 2011
psu- CORSAIR Enthusiast Series TX750 V2 750W
cpu- Intel Core i7-3820 Sandy Bridge-E 3.6GHz (3.8GHz Turbo Boost) LGA 2011
ram- G.SKILL Ripjaws X Series 8GB (2 x 4GB) 240-Pin DDR3
HD- Seagate Barracuda ST1000DM003 1TB 7200 RPM 64MB Cache ---> should of installed os here but chose wrong one in setup
cpu fan- CORSAIR Hydro Series H100i Water Cooler
case- Corsair Carbide Series 400R

i'v read countless reviews on the gpu and never sew my problem and searched and all solutions i found didn't work either