There is a lot of false information in these answers about "theoretical" performance from people who have evidently never benchmarked USB2 HD transfer rates.
I have benchmarked many different USB2 transfers between 2.5" laptop HDs both PATA and SATA, 3.5" HDs both PATA and SATA, and USB Flash drives...
...and I have NEVER seen transfer rates exceed 35 MB/sec! In fact, any properly configured modern drive will transfer at 20-30 MB/sec, it's very rare to see the 30 MB/sec rate be surpassed. (I'm referring SPECIFICALLY to HDs transferring over USB2 here, to be clear.)
Ignore this talk about theoretical transfer rates and "60 MB/sec", etc. Although I give credit to the guys who correctly converted bits into bytes and calculated a 35 MB/sec maximum, which falls in line with my REAL WORLD PERFORMANCE EXPERIENCE.