Had access to a 1TB WD drive recently. Comes with USB2, FireWire 400 & FireWire 800. Thought I’d check out the performance of the various connection methods. It had two internal 500GB drives arranged in RAID 0 (striped). I tested it by running:

$ time cp Movies/Parallel.avi /Volumes/MYBOOK/
The movie was 525MB and I did each test 3 times. The very first run was slowest – presumably the file (or parts of it) was in the disk cache for subsequent runs. Here’s the times:
FW800: 12.5 seconds
FW400: 16.9 seconds
USB2: 21.6 seconds
Just to make sure my laptop drive wasn’t affecting the test, I also performed this test (several times for each):
dd if=/dev/zero of=/Volumes/MYBOOK/test.file.fw800 bs=1m count=1024
FW800: 24.8 seconds
FW400: 32.4 seconds
USB2: 40.9 seconds
For comparison:
 Local SATA drive: 20.3 seconds
My laptop hard drives results were a bit erratic – peaking up to 27 seconds and down to 20. No doubt due in part to the 21 applications I’m currently running. Stopping iTunes playing helped things ;-)
I was surprised that FireWire 400 was that much faster than USB2 – I’d always assumed they were on par. Anyway, looks like FW800 is clearly the king for connecting external HD.