How to use Terminal to securely erase free space on a Mac’s mechanical 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).Editor’s note: This Terminal tip originally ran in March 2009 and is only useful for mechanical hard drives and not the SSDs found in newer Macs.When you click this button, you’re pres