82GB of “other” don’t appear in OmniDiskSweeper or Freediskspace

Ha, found it! It was a corrupted ".Trashes" folder in the root of the SSD. It occupied me than 100GB of data (see screenshot).

I managed to make these files visible by running GrandPerspective from the command line:

sudo GrandPerspective.app/Contents/MacOS/GrandPerspective

That will show the files but still it is not possible to delete them. Even

sudo rm -rf /.Trashes/*

did not work. It just returned immediately without any output. The solution was to have Terminal run as root and then delete, so start Terminal and run:

sudo /Applications/Utilities/Terminal.app/Contents/MacOS/Terminal

Then, in the "root" Terminal window:

rm -rf /.Trashes/*

Running GrandPerspective again afterwards shows only 16MB of miscellaneous space.

If you know the name of one of the movies (files) that you are looking for or if you know what format the movie is in (extension) then you should be able to use Spotlight search

My favorite way to bring it up is hit Command-Space and you will see a search bar in the middle of your screen (Yosemite)

Start typing the name or file extension and if it exists, you will start seeing search results pop up.

Select any of the files and hit Command-I for Get Info and you should see a window like this pop up:

Under the "General" tab, you will see a field labeled "Where". This is the path to your file where it's located. In this example, the location is /Applications.

You can now use Finder to "Go to that folder". Hit Shift-Command-G and type that location into the dialog box. A Finder window will open, at that location. Your file(s) will be there.

As for names - well, you will be the one who knows what they are. As for the extensions, there are many, but some of the most common are .MP4, MPEG, and MOV. for a complete list, check out Video File Extensions