Which SATA port do you have it plugged into? There are three controllers: native Intel SATA 6 Gbps, Asmedia SATA 6 Gbps, and Asmedia Sata 3 Gbps. I'm not sure on the performance difference between the Intel and Asmedia 6 Gbps ports, but the 3 Gbps port will sure ruin your day. I am getting about 550 R/W on the native Intel ports with my Samsung EVO drive..
Rampage IV Black Edition | 4960X 4.5 GHz x6 @ ~1.232V | 64 GB 2133 1.5V @ Stock XMP
2x Crossfire Matrix Platinum 7970s @ Stock | AX1200i | H100i | Corsair Link Commander
1 TB EVO | 3 TB WD Red | CoolerMaster Cosmos II