Next I'll briefly discuss on a similar approach by quoting relevant parts about two other things that can be done to look for missing disk space, without installing anything new, just using the command line (the Terminal).
In most cases, there really are files occupying part of the volume, but the files are invisible in normal use of the Finder.
Using the Finder’s Go to Folder feature (in the Go menu), look at the sizes of the contents of these folders, by pasting in these pathnames:
The /private/var/vm directory contains the swapfiles used by virtual memory. New ones are made as more data is swapped from RAM to the hard drive. The entire process of creating them begins at each reboot or restart; do not attempt to remove them yourself. Check the total size of all the swapfiles, right after you boot, and as the disk fills up. In Panther, the first two swapfiles are 64MB, then each new one is twice the size of the preceeding one (128MB, 256MB, 512MB, 1 GB) up to a maximum size of 1 GB. In Tiger, the first two swapfiles are 64 MB, the next one is 128 MB, and any additional swapfiles are 256 MB.
If you do not run the daily, weekly, and monthly maintenance scripts (either by using a utility, or by running the commands sudo periodic daily, sudo periodic weekly, and sudo periodic monthly in Terminal), the logs on the startup volume can become too large. If an error is occurring frequently and is being logged, you can have a very large file at /private/var/log/system.log.
The files in /Volumes should be aliases to your mounted volumes. Do not remove these aliases, because anything you do to them happens to the contents of the corresponding volumes. If you are not confident that you can explore this folder without mishap, before you begin, properly unmount any volume other than the startup volume, if the missing disk space problem affects only that volume. External FireWire drives can be disconnected after proper unmounting.
Sometimes, backup programs that cannot find an intended destination (or target) volume for a backup create a folder with the same name as the destination, and put the folder into the /Volumes directory. There are cases in which the entire startup volume has been backed up on itself, in a folder inside /Volumes. If the amount of missing space is about the size of your user folder, such a backup is likely to be the explanation. If you use Carbon Copy Cloner or another backup or cloning utility and have its preferences configured to create a backup on a schedule, and the intended destination volume is not mounted or is sleeping at the scheduled time, the backup is created in the /Volumes directory.
To check the size of the normally invisible /Volumes directory on the active startup volume, open a new Finder window. Select the startup volume in the list at the left, then choose column view (the one at the right of the three views). From the Finder’s Go menu, choose Go to Folder, and paste in:
The /Volumes directory becomes visible in the Finder; find its size by selecting it and typing Command I. My /Volumes directory is reported to be 12K.