The history of USB on Macs, I guess, is becoming a little cloudy. USB made its first appearance on Windows machines... that should tell you a lot right there. Macs were SCSI devices (for HDs and other "high speed" peripherals, like scanners.) Apple went out if its way to accomodate would-be converts from Windows by adding USB, and the "it just works" era was begun. For the most part, that's still true, and, USB drives will work with any software you can run on a Mac. That being said, USB is like a CB radio: a one-way-at-a-time stop-gap communicaitons protocol. It's never going to change from that... too much legacy windows crap around that requires it. Every USB drive is an *end node* - doesn't matter how many there are or how many are attached to hubs, how much storage expansion you've accumulated, they are not lined up on a bus. USB 2, USB 3, and beyond, it doesn't matter how fast it gets, it's still going to tie up the system and the software while the commucation is negotiated, and that is just not conducive to the way video editing software has been written.
Firewire on the other hand is a peer-to-peer communcation protocol. The system can throw off a lot of the responsibility of reading and writing data to the external machine and that external machine can "talk to" other external machines on the bus without having to interact with the system to manage the data flow. Firewire is the child of SCSI and it was designed *specifically* for audio/video applications. USB was not; it is more a general purpose protocol. It will work in a pinch.
You own a Mac because it is the best (consumer) video production machine, but you're using Windows technology on the back end. It's kinda like putting a Ferrari body on a Volkswagen. It will serve your purpose, for now. Eventually, you're going to want the performance, not just the ride.
For the last few days, I've been kicking myself. I *have* a FW 800 drive and did NOT realize until about 5 days ago, that yes, you can actually MIX 800 and 400 drives on the same chain. I had been under the impression for the last 2 years (since I purchased my iMac with FW800) that any FW400 drive on the chain would slow the entire chain down to the slowest drive. ** I was wrong.** If you have a mix of FW 400 and 800 drives, connect all your 800 drives first in the chain and start them in order of connection. Place all the FW 400 drives at the end of the chain and start them up "last". All the drives will be accessible and the 800s will work as 800s along with the 400s. I can expand my storage and not lose my legacy drives! That, in itself, will save me a ton of money.
Thanks to this discussion for lighting the fire(wire) under me! The extra research paid off.