trying to understand disk utilization

Hi

I'm fairly new to mac, so excuse me if I ask something really stupid.
I have a macmini (80GB HD) with an external USB HD of 160GB. The external drive has 2 partitions and on one of them (size: 100GB) I make time machine backups, but the partition also contains a few other folders. The finder tells me this partition, has only 26GB available space. looking at the size of the folders, there is only something like 30GB used, so it seems 44 GB is not accounted for! Reading through forumposts made me type the following in terminal:

MacMini:macdata ZAdmin$ sudo du -h -d 1
Password:
2,2M ./.fseventsd
215M ./.Spotlight-V100
0B ./.TemporaryItems
0B ./.Trashes
du: ./Backups.backupdb/MacMini/2009-08-23-213942: No such file or directory
815G ./Backups.backupdb
363M ./Downloaded Programs
8,3G ./IphoneMovies
1,2G ./Temp
825G .

So that seems to suggest my backups take up 815GB of space !! Not bad for a 160GB disk.... Am I overlooking something or is there something corrupt on the drive?

Thanks !