I have a Western Digital My Passport Studio external hard drive. It works with either Firewire 800 or USB 2.0. I've noticed that when I have it connected to Firewire, after a few hours, processes on my Mac start to go into an uninterruptible wait state. Eventually the system becomes so hard locked that I can't even shut it down. I have Spotlight indexing of the drive disabled, and the mds process seems to be the one that triggers this eventual system collapse.