Here is how to open and use DiskPart using Command Prompt: Search Command Prompt in the Windows search bar and launch it with admin rights. In the command window, type diskpart and hit enter.

After entering the last command, you will be able to use the external hard drive or USB drive regularly. TIP: You can format USB Drive using Command Prompt as well.

How to Format an OS X Terminal Hard Disk. Mac OS X Mountain Lion gives users several ways to access the Terminal and three common options for formatting a hard drive.

I know some programs can do it (any free ones?), but I thought there was some sort of low level format command or whatever option that would ensure someone couldn't 'undelete' your files.