SATA II is also called 3 Gbit/s. That is 300 gigabits per second. If you take 300 gigabits per second and turn that into megabytes per second. Mathematically, with overhead, this brakes down to about 300 megabytes per second max.
However, there is no mechanical single spinning drive that can hit 300 megabytes per second. In fact, nearly all SSD's cannot go that fast either. About the best that you will get with a standard 7,200 RPM spinning drive is around 100-120 megabytes per second.
I've got some WD Caviar Blue's at work and with benchmark software (Sisoft or HDTune) they get about 85 megabytes per second.
If you copy real world files though from the device and to the device....expect things to slow down as it's both reading and writing to the same device. And depending upon whether they are large contigous files or tons of small files will also impact your speeds.