This article discusses the different speeds of the SATA interface.
As of the writting of this article, there are three SATA specifications.
Specification 1 specifies that speeds up to 1.5Gb/s (187.5MB/s) is possible.
Specification 2 specifies that speeds up to 3Gb/s (375MB/s) is possible.
Specification 3 specifies that speeds up to 6Gb/s (750MB/s) is possible.
One must remember, these speeds only represent the maximum bus frequency from the controller to the device controller. There are other factors to consider such as the speed of the mechanical operations of a hard drive.