1. Empty the Trash.
2. If you use iPhoto, empty the trash within the program.
3. If you use a Mail program, delete the trash.
4. If you are backing up with Time Machine, some of the space may be local snapshots.
5. If you have a large iTunes and/or iPhoto collection, consider moving them to you external drive.
Also, get OmniDisk Sweeper and run as root to see other users. (Be extremely careful what you delete there because it will possible to cripple your Mac.)
To run ODS as root.
Open Terminal and copy/paste and hit return. Give your admin pword when requested, which won't appear as you type it in. Hit return again.
I think the problem is Quick Time Player. I am using OS 10.6.8 by the way.
I tried it out again.
I had about 26 GB of space before I played an .avi video.
After the movie started to play (it was slow to load), I got a warning message that my startup disk was full. I opened Disk Utility and it told me I had no space left (about 20 MB or something like that was left).
I watched to the end of the movie.
Then I quit Quicktime Player.
And I shut down the computer.
I turned on the computer and checked Disk Utility. I have about 26 GB of space.
I think for some reason Quicktime Player uses up a lot of the disk space when it runs movies that are problematic. This movie I was watching did not open with vlc.
I normally use vlc to watch movies.
Probably it doesn't use up this much space when the movie is OK.
I think there's a problem with this particular movie. It is about 50 MB so it's not that big a file. I watch .smi subtitles with it. The Quicktime Player automatically loads the subtitle file in this case because the subtitle file has the same name as the movie file.
I watched about 10 movie files from the same series that I downloaded together and did not have any problem watching them. I watched them on vlc. But this latest movie had problems I think (as I repeat). It did not load on vlc even though I waited for a minute, and normally movies load instantly on vlc.
So I think that movie file caused problems. It made QT player use up a lot of hard disk space.
I had better avoid watching that movie in the future. I don't want it to wreck my hard drive.
So to sum up, everything is back to normal. I have the hard disk space that I should have. However, I am scared of damaging my hard drive due to shonky movie files that eat up all the space when they are played using QT player, and so I will not watch those files in the future, as I do not know why the files do that.