1) USB2 bus is 60MB/second
2) USB3 bus is 600MB/second
3) USB2 hard drive is usually about 32MB/second (slow interface chip)
4) USB3 hard drive interface speed is ???
My guess is that most USB3 hard drive interface chips will be at least as fast as the hard drive's maximum transfer speed which is usually about 120MB/second.
So how much difference would it make to have a USB3 hard drive run on USB2 instead of USB3?
In practice, not much difference.
Most smaller files would fall below USB2's 60MB/second speed. Since the maximum speed (on the outer edge) of the hard drive is about 120MB/second for larger files the worst case scenario is that it would be 2x slower on USB2 than USB3.
*USB3 thumb drives which use SSD technology are different. Within a year from now they will reach 600MB/second. One was just released at 280MB/second.