ATA100 (HDD) = 100 Megabytes / second
USB 1.1 = 12 Mega BITS / second = 1.25 Megabytes/second
USB 2.0 = 480 Mega BITS / second = 40 Megabytes/second
Those are theoretical maximum speeds too. So you will get slightly less.
HDD is always faster than USB in raw transfer speed. The gain for USB comes from the tiny seek times compared to spindle HDDs, because flash disks have no spin up - it is solid state. If using a normal HDD over a USB connection you must wait for the HDD to seek/spin up AND you get only a maximum of 40MBytes/second throughput.