Theres an issue with the nvidia MCP79 sata controller on your board that apple/nvidia never bothered to fix. Basicly a sata 3 drive works at sata 1 speeds. I had this problem on my OCZ agility 3, the only way to resolve it was to use OCZs tool to set the drive to work at sata 2 speeds.
I was actually thinking about this and I remember reading a couple of years ago(when I set up my SSD) that this issue only affected a few SSDs so try updating the firmware and if not get in touch with samsung and limit the drive to sata 2
Im actually really annoyed people are still having problems with the nvidia MCP79 controller and ive already emailed samsung technical about the problem, waiting to hear back for their solution.