this happened to me too. when i saw the free space getting close to 0, i hit stop, but it did go all the way to 0. i rebooted onto the windows side of my computer and ran WinDirStat, where i saw that everything was in the private/var/folders directory ("private" is a hidden folder). that directory had 2 directories in it, named hm and zz. it looks like those are a lot of temporary and cache folders/files. inside of one of them (sorry, don't remember which) there were a couple of files that added up to the free space i had lost. i deleted those 2, rebooted into osx, and my free space was back.
hope that helps...