I am on Snow Leopard btw. I have a 1 TB external HD that I'm using with Firewire 400 and I've noticed that it seems to slow down my computer. Like for instance I'll boot up my computer and it will take forever for it to start up. Then, as soon as I turn off my drive it boots up immediately. Or I'll use Spotlight and then basically it gets stuck with a spinning beach ball and as soon as I turn off the drive everything is fast again.