If you want to find out where your disk has gone run the following command on your shell
du -h /usr/ | grep ^[0-9.]*G | sort -nr
This will find all the directories with GB s
now repeat for other directories until you find the problem
du -h /var/ | grep ^[0-9.]*G | sort -nr /var is usually the biggest pain. Also run it for /home
du -h /home/ | grep ^[0-9.]*G | sort -nr .