Where have all the bytes gone?

Here's an "interesting" phenomenon I noticed on my new (not-Linux-hating) 4GB portable music player:

daniel@jeeves:~$ df -h /media/IAUDIO
Filesystem Size Used Avail Use% Mounted on
/dev/sdb 3.8G 3.8G 0 100% /media/IAUDIO
daniel@jeeves:~$ du -hcs /media/IAUDIO
2.3G /media/IAUDIO
2.3G total

Just in case "df" and "du" had different notions of how large a gigabyte was, I asked them to tell me the number of bytes instead, with about the same result.

I won't comment on the fact that I paid for 4GB but got 3.8GB; that's typical (whoops, I just did). But where'd that extra 1.5GB go? FAT can't seriously be imposing a >50% overhead on file size, can it?

The end result here is that on my 4GB music player, I can store only a little more than 2GB of music. Boo. :-(

[UPDATE]: running fsck revealed that I had 1.5 GB in lost clusters on this filesystem and gave me my space back. Since this device is almost brand-new, I'm puzzled about how this happened...but oh well, it seems to be fixed easily enough.