Overwriting “unused space” on Windows 7, Windows 8, and Windows 10 is quite simple. Open a “Command Prompt” window, and type:
Where “DD” is your drive without a suffix, e.g.
to wipe unused space on drive C.
Why is this useful? Well, when you delete files from most modern operating systems, they aren’t really erased, even after you “Empty the trash”. The file system on your drive is simply updated to indicate that the space previously occupied by the file is now available. But the file data is still there. Overwriting such “unused space” with nonsense/garbage data will make it harder to recover the file data.