How much of a transfer rate difference does SATA 6GB/s vs 3GB/s make?

[Legolas8181]

I am looking at motherboards for a new build and I am finding a lot of boards with two SATA III slots and 3 SATA II slots. I would use the SATA III ports for my SSDs obviously but then if I use one of the SATA II ports for my HDD how much speed going from say that HDD which is 7,200RPM to my external HDD which is USB 3.0 and 7,200RPM?

 

[ewok93]

6GB/s is twice as fast. Not sure what else you're trying to say.