Why not USB??

Question

Hi --
So it's widely understood that USB external drives are no good for editing on a Mac. Borrowed quote from an earlier post that I found:
"Recognize that USB is not recommended for use as an editing drive - this is why: while capturing is a relatively simple 3.6MB/sec write, editing involves multiple file reads at the same time which requires a much higher sustained through put - a sustained effort that USB can not reliably provide. If your editing is simple cuts only with one track of video and not very long, you may get away with it but the odds are decidedly against it working without dropped frames, stuttering and so on."
But if thats the case, why does USB work fine with PC edit systems??
I was wondering if anybody can explain once and for all why PC's can edit with USB external HDs but Macs can't? I've searched a lot but cant find the answer to this..

Answer 1

This information is from the APTS book "Optimizing Your Final Cut Pro System" Page 386:
USB
The Universal Serial Bus is in many ways the successor to the older Apple Desktop Bus: a serial bus for the connections of various accessories, like mice, keyboards, and even storage devices. USB drives, like FireWire drives, aren't USB-native, but use bridges between ATA or SATA drives and the USB connection running back to the Mac.
USB 1.0 and 1.1, the original versions, top out at 12 Mb/second, for a piddling 1.5 Mb/second transfer speed. Hi-Speed USB 2.0 runs at a more respectable 480 Mb/second, giving it 20 percent more bandwidth than FW400, at least in theory.
In practice, USB suffers from being a CPU-driven protocol; the controlling computer has to be much more involved in data transfers over USB than over FireWire. Thus, sustained transfer rates tend to be slightly lower on Hi-Speed USB 2.0 than on FW400. And that's in the best case, when a fast computer is being used and isn't doing anything else; if the Mac is busy doing other things (like rendering a real-time effect and displaying the output in the Canvas), performance falls off even more.
Hi-Speed USB suffers just like other multiple-speed buses do: plug in one 12 Mb device, and the entire bus slows down to 12 Mb. Fortunately, each USB 2.0 port on the Mac has its own internal hub, so you can segregate 12 Mb and 480 Mb devices from each other just by plugging them into different ports.
USB is fine for offline data backups and for moving projects from system to system, but it's simply not efficient enough to be the interface of choice for media drives. Although some brave souls have made Hi-Speed USB 2.0 drives work for DV editing without seeing too many dropped frames, it's just not robust enough for video work, especially not when FireWire is available.