First try a different SATA port, or/and SATA cable.
Might not be the cause, but can be one. I had a HD that was always 1/2 the speed of my others. I just chalked it up to it being a 1gig 5200rpm cheap drive I got off newegg.
After doing some cable management one day dusting out my case, I decided to replace some of my SATA cables with shorter ones I had since it looked better..and that drive sped up a LOT on transfers. I put the old cable back, and it slow again.
So something with that cable was bad but not bad enough to stop working.
-We don't control what happens to us in life, but we control how we respond to what happens in life.
-Hard times create strong men, strong men create good times, good times create weak men, and weak men create hard times. -G. Michael Hopf
Disclaimer: Post made by me are of my own creation. A delusional mind relayed in text form.