I use an eMac with four external FireWire hard drives. I capture video daisy-chained with the camcorder plugged in after two hard drives. But I capture to my internal hard drive. After I'm done capturing, I move it to an external drive for editing. I've found that trying to capture directly to the external hard drive occasionally causes errors. (Dropped frames, video glitches, iMovie just deciding to stop importing while the camcorder keeps playing, etc.) This is true wether I have the camcorder on the same chain as the 'target' drive or a separate chain. (So in my layout below, it would be true if I'm capturing to HD1 or HD3.)
I do have a USB 2.0 external hard drive that I occasionally connect, but I've never tried capturing video directly to it, so I can't comment on how well that works. Theoretically, it should work just fine. But, theoretically, so should capturing to a FireWire drive. If there were a way to prevent iMovie from showing the live feed during capture, it should be able to run perfectly smoothly to an external drive. (Since FireWire allows for transfers directly from one device to another without passing through the host PC. But since iMovie wants to show the video, it HAS to pass through the host PC.)
This is my normal setup:
eMac FW1 ------ Hard drive 1 ----- Hard drive 2 ----- camcorder
+- FW2 ----- Hard drive 3 ----- Hard drive 4 ----- iPod