I back up my PS3 to a USB drive, which has a FAT32 partition. The archive folder reported only 3.2GB, but there was about 7.2GB in use.
The Finder and du both reported 3.2GB. I finally tracked it down to a file that is 4GB, but reports as 0. Most of the time. Note that even ls -s gets it wrong, but ls -l gets it right.
$ du archive2_00.dat
$ ls -s archive2_00.dat
$ ls -l archive2_00.dat
-rwxrwxrwx 1 pudge pudge 4294966784 Feb 21 22:21 archive2_00.dat
The Finder was similarly confused.
Cross-posted on <pudge/*>.