Recently I had to share an external FAT32 formatted hard drive with another Mac, only to discover that OS X (10.4 and 10.5; I don't know if previous versions did the same) don't share non-HFS partitions.
Googling around, I found some posts here and there discussing the problem, but no workaround (apart from reformatting the drive using HFS+). In brief, I found that sharing FAT32 (and NTFS) partition is possible and not so hard to do; you just need to use Samba.
[MyFatShare]
path =/Volumes/myfat32hd
public=yes
writable=yes
browseable = yes
only guest=yes
Then save the file (in pico, you can type Control-X, then answer Y). (You can tailor the share permissions to your liking. In this example, I created a public world-writable share).Mac OS X Hints
http://hints.macworld.com/article.php?story=20090216180227216