A few weeks ago when I booted up my home iMac my firewire drives were curiously absent from the desktop. It was odd since they'd been working the night before. Fortunately, my system is still covered by Applecare so I called for support. Turns out, if you unplug your computer and let it sit for a good thirty-seconds, the firewire bus will clear. When I rebooted my drives were back.