In addition to erase and partition, your Mac has four Secure Erase Options in the Disk Utility. Don’t Erase Data. Zero Out Data. 7-Pass Erase. 35-Pass Erase. 7-Pass Erase meets the US Department of Defense 5220-22 standard to wipe a hard drive. If that’s not good enough, try Chris Breen’s manual method.
Allow me to propose the Multi-Pass Sledgehammer option. If you need your data to be totally unrecoverable and are willing to sacrifice a hard drive to make that happen, extract the drive from your Mac, take it out back, and beat the living hell out of it.
Funny, yet effective.