Use The Recovery Partition to Securely Erase Your Mac’s Mechanical Hard Drive

Here are the steps to securely erase your spinning hard drive on OS X 10.7 or later:

1. Turn on the laptop and immediately press and hold down both the Command key and the R key. This instructs the computer to startup from the Recovery Partition.
2. Once you see the Apple logo appear in the middle of the screen you can release the Command and R keys.
   
3. Give your Mac a couple of minutes to startup, at which point you’ll see a window like the one this one.
4. Click on the Disk Utility option from the “Mac OS X Utilities” window.
5. In the left pane of the Disk Utility window, click on Macintosh HD (which is your laptop’s internal hard drive).
6. In the right pane of the Disk Utility window, click the Erase tab.
7. Click the Security Options button.
8. I recommend that you select the 7-Pass Erase options. I think you might just see a slider, in which case you slide it to the right side labeled “Most Secure.”
9. Click the OK button.
10. Click the Erase button.
11. You’ll be asked if you’re sure you want to erase it. You’re sure. Go ahead and click the Erase button.