I tried a USB2 external drive on my MacBook Pro - it was a 5200 RPM, bus powered model; inexpensive, small, cool - but had audio drop outs (running BFD1.5, Kontakt, Battery 3).
A while back I got a 7200 rpm glyph GT model that has USB2, SATA, and Firewire connections (both 800 and 400). I've been using that through Firewire 400 with no problem. The gylph seems rock solid, though it was somewhat pricey. It is not small or bus powered.
Maybe a 7200 RPM model would work for you through USB2.