Just got a Firewire/IDE enclosure. Put in a 60 gig drive. Since I want to use this drive on both OS X and Windows (2k), I formatted it as Fat 32 (which both OSes know about and can mount). Problem one was that I couldn't create a Fat 32 partition that big, so I split it up to two 30 gig partitions.
Now, I formatted these partitions in windows. The wierd thing is that both Windows and OS X ignore the labels (so it seemed). During the course of figuring this out, I ended up (at one point) with both of the partitions on my FireWire drive having the same name. This was OK on Windows, but on OS X. It would mount the first partition, but not the second. It couldn't even properly determine the filesystem type of the second (although I could mount it via the Terminal, Finder wasn't automounting it). Plugged the drive into Windows, changed the partition name (no reformat necessary), and voila, it worked again in OS X.
Very obscure and wierd, but it might happen to you (and it's probably a bug, because I could mount it via the shell).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020128083128489