Mac Mini 80 GB HD says it's full with only 25 GB of data...


I'm new to the forum and hope someone can help me out - (BTW I did a search for "Mac mini" but got a blank screen in my Firefox and Safari browsers - is there something going on with the search feature on the forum site?)

I have a friend with a Mac Mini, 80 GB HD. The drive information under "Get Info" says that the drive has something like 78.5 GB of data on it, but there is actually only about 25 GB of data on the drive. This has us both stumped. He tried to run the disk utility to repair it and it basically runs and says the disk is OK but the same thing keeps happening. So we're stuck. The disk won't take more data, even though there is plenty of room.

There is a Maxtor 180 GB external Firewire drive hooked up to this machine. There are multiple iTunes music libraries on that external drive, under different accounts on the Mini, but the music is all stored in a single volume on the external drive so that everyone can access it. This is the only thing I can think of that might be causing some kind of funky problem, but I'll be damned if I can figure out what it is.....

He is running Tiger but sorry I don't know the exact version right at the moment....but the machine is relatively new (as in bought in the last 6 months....)

Just thought I'd throw this out there in case anyone has an obvious answer before we go further - like we might have to buy Disk Warrior, etc.....not sure where to go with this. Any suggestions/help would be appreciated.....thanks!
first thing i'd do is download and run OmniDiskSweeper, or the like, to see if there is a huge hidden file somewhere. i got a mac+ for free along time ago because of that. it had only 5mb on its 20mb hd, and said the drive was full, so i did some hunting and found a 15mb spooler dump file. deleted that and then i could now have a second computer to play robosport on :D . also try MacJanitor to make sure your log files and cache have not grown out of control.