In some installations of OSX ie. Mavericks and Yosemite, when you delete files, the space changes to reserved disk space.

What actually happens,  the files get moved to a hidden folder on disk root called /.MobileBackups

Before you can clean the .MobileBackup you need to have a password set on your user account. You can do this form Systemsettings.

Start the terminal.

 w

sudo su
cd /
ls -l
du -chx -d 1 .MobileBackup
rm -rf .MobileBackup
exit

Before deleting the folder .MobileBackup, make sure you are certant there are no files which you do not want to backup.
Go through the folders with “cd” and “ls -l” command

When doing the “du” command, it might take some time depending on the disk size and speed.