It is possible to do this sometimes on older systems with a software workaround that is provided by the hard drive manufacturer. Maxtor has their EZ-BIOS software that allows hard drives that wouldn't normally work with something like FAT32 to be fully operational and have no limits. Of course, once that software is taken out of the picture, the limitation appears once again, and make the drive unmountable or unreadable unless said software is in place.
If it is FAT32, I recommend that you check the machine that it was installed in. Start it up and see if the EZ-BIOS is installed in the MBR (Master Boot Record) of the drive. It will show up on boot. If so, then this could be the reason and you will probably have to move the data from that drive to another computer, reformat it under the Mac, and then bring back your files.