How to use Terminal to securely erase free space on a Mac’s drive

If you’re selling an old Mac, a spare hard drive, or you’re just quite paranoid about your deleted data, you’re either familiar with—or should be familiar with—the Erase Free Space button on the Erase tab in Disk Utility (found in your Applications -> Utilities folder).When you click this button, you’re presented with three options for securely erasing the free space on your hard drive: write over the free space with zeros (fast and relatively safe), write