Repair the Hard Drive and Permissions - Lion
Boot from your Lion Recovery HD. When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported then click on the Repair Permissions button. When the process is completed, then quit DU and return to the main menu. Select Restart from the Apple menu.
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Use a tool such as OmniDiskSweeper to explore your volume and find out what's taking up the space.
Proceed further only if the problem hasn't been solved.
ODS can't see the whole filesystem when you run it just by double-clicking; it only sees files that you have permission to read. To really see everything, you have to run it as root.
First, back up all data if you haven't already done so. No matter what happens, you should be able to restore your system to the state it was in at the time of that backup.
Launch the Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ If you’re running Mac OS X 10.7 or later, open LaunchPad. Click Utilities, then Terminal in the page that opens.
After installing ODS in the Applications folder, drag or copy — do not type — the following line into the Terminal window, then press return:
You'll be prompted for your login password, which won't be displayed when you type it. You may get a one-time warning not to screw up.
I don't recommend that you make a habit of this. Don't delete anything while running ODS as root. When you're done with it, quit it and also quit Terminal.
I just had space disappear and could not for the life of me figure out where it went to. I tried all the tips mentioned here and elsewhere, and could not discover where I lost it....
Turns out I had copied files off the laptop using FireWire target disk mode, then deleted them but FORGOT TO EMPTY THE TRASH! They showed up nowhere, and no disk utility revealed the files or where they were, until I rebooted it again in target disk mode and emptied the trash...