Transfer speed of USB 2.0

I have a question.

USB 2.0 transfer speed is 480Mb/sec.


Currently I have a 320GB internal hard drive and my friend recommends an 
external 320GB for backup.


I told him the transfer speed (based on a full 320GB of information) is
too 
slow and tried proving it with math isn't working.


480Mb/sec is equal to 60MB/sec or 216x10^9 B/hour.


canceling the powers: 320/216 equals 1.48hrs to transfer all that data.


I just backed up my main internal 320GB (about 250GB worth of data) to my 
other internal 1TB hard drive and it took about 2-3hrs. I'm not sure of 
internal data transfer speeds, but would think it's faster than USB.


How could an external drive be faster than my internal drives?


TIA