I've got a reasonably old Planet server with a single 80G disk, but apparently i've only got 7G left on it which is quite worrying.

I did "du -sch /" and it only says 33G was used, so where was the other 30+ going? I took a bit of a gamble and rebooted the machine, and as a result suddenly my disk usage is down to 46%. Wtf?

I did lots of selective du-ing first on the directories i knew contained lots of stuff, but i could only ever account for roughly 30G.

Where was the other 40G+ going? Any suggestions would be hugely appreciated.

You may need to run an fsck. When the two don't agree, du is usually the more accurate.

This happens if someone or something deletes a file which is being written to, often happens when people delete or move /var/log/messages without shutting syslog down first.

Just make sure you have a good set of backups before having the file system checked... Just in case the missing space is due to orphaned inodes...those poor orphans!

It is very possible that this data is 'hidden' under another mount point. One of the common issues we see is where a person has '/backups' mounted to store their backups, but for some reason, during one of the backups it doesn't mount. The files are then written to '/' in a folder called 'backups', which is then hidden the next time the partition re-mounts. You may want to unmount any 'un-necessary' partitions and make sure they aren't hiding data.