Hard drives can be SATA 6Gb/s compatible, not 6Gb/s capable.
No hard drive can spin fast enough to achieve SATA 6Gb/s speeds.
Your drive has a buffer (16MB, or 32MB, or 64MB) which briefly transfers data at 6Gb/ speeds, but that's it.
What the model of your drive?
It's the WD1002FAEX drive. I have it hooked up to a SATA 6Gb/s port.