I have had a ongoing problem with miscellaneous FireWire drives that won't mount. Clicking on the grayed-out drive in the Disk Utility only creates an endless beachball. After trying DiskWarrior, Data Rescue, and other various tools that were no help, I tried booting into OS 9 (sorry newer Mac owners) and the drive immediately showed up on the desktop. When I booted back to OS X, all was well and the disk just worked.
Next time it happened, I just started Classic and the drive came right up. I don't know how or why it happens, but it has worked with other clients' computers, as well.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040827005041917