In order to wipe Free Space, you need to have Administrator write access to the root directory (/). If you are the only user on the computer, you should have that by default. If there is an Administrator for the computer, - Log in as the Administrator - Run the Disk Utility and Repair Permissions on your startup disk - Run ShredIt again. If its still not working, then - launch the terminal and type this command: sudo chmod ug+w / - You will be prompted for the admin password. - Then run ShredIt again. If following these steps doesn't solve the problem, contact support.