After an app (VirtuFrame) crashed my Mac Mini, I discovered upon rebooting that my LaCie external FireWire drive was no longer visible. I disconnected and reconnected the cable several times, to no avail. It wasn't visible in either the Disk Utility or the System Profiler. I then reformatted the drive, but still no luck (good thing I didn't have any irreplaceable data on there). But when I connected it via USB cable, it showed up immediately. Apparently, the crash had somehow hammered my Firewire IO subsystem. After connecting via USB, I tried updating the FireWire firmware on the drive, but it still wasn't visible.
Based on advice I gleaned from a number of forums, I then tried the following: I shut down the Mini, unplugged it, and let it sit for five minutes -- this flushes the volatile RAM. When I rebooted, the drive was visible! Another suggestion is to wipe the PRAM: reboot and hold down Option-Command-P-R while booting (before you see the grey screen) and until the login screen comes up. After logging in, the drive should be visible.
If you're still stuck, search the Apple site or other locations for "Firewire bug."
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050825040102452