How to free up memory on a Macintosh HD with OS X?

Well, that means someone has a lot of files on the computer. Try to find which directory in Users has all the files. If you can't find it, open up /Applications/Utilities/Terminal.app and type (without the parenthesis) "defaults write com.apple.Finder AppleShowAllFiles true". This will let you see hidden folders. If you want to hide the folders again, type the same thing but replace true with false.