Missing Disk Space

Are there other user accounts that might contain large amounts of data?

The system-supplied subfolders (Documents, Desktop, Music, etc) in a user's Home folder have restricted permissions, and can't be accessed by a different user, even an admin user. They will be reported as size zero by Finder and by programs such as Disk Inventory X or OmniDiskSweeper when measured by a different user, and this will make the summary info incorrect as well.

Launch the Terminal app in /Utilities, copy & paste this command into the window that pops up, hit the return key, and post the output:

df -H

Then, launch Activity Monitor, also in /Utilities, select Disk Usage, collect the used and free data from each mounted volume, and report those results.

Glad it's straightened out!

As an aside, I see that the df -H Terminal output you posted earlier showed incidentally that you had earlier downloaded the WhatSize .dmg and mounted its disk image:

/dev/disk5s2 42M 23M 19M 55% /Volumes/WhatSize 10.4.9.10

For others who might be interested, the paid ($12.99) version of WhatSize has the ability to simultaneously measure the sizes of all items on the disk, restricted or not, because it can scan a volume with "root" privileges, a feature that freeware utilities such as Disk Inventory X and OmniDiskSweeper do not offer. To enable this feature, one would click the "As User" padlock at the top of the WhatSize window and authenticate when prompted.

Usually this WhatSize feature, though very convenient, is not really necessary. The combination of using a freeware utility and also logging into other user accounts will find most instances of "missing disk space." There are also other ways to measure disk utilization using "root" privileges that don't cost anything, but they require some tinkering.

Could be the startup disk needs repairing or replacing.

Run Disk Utility.

Using Disk Utility to verify or repair disks

Have you emptied the trash lately?

http://www.thexlab.com/faqs/freeingspace.html

Your Macintosh Harddrive disk

/dev/disk0s2 250G 149G 101G 60% /

matches

Macintosh Harddrive utilized: 148.68gb free: 101.03gb = 250gb

So, nothing's missing.