How to Securely Erase a Disk

This is in response to a query from a reader. They want to know how to securely erase a hard disk so they can dispose of the old Mac.

The short answer is that you can’t erase your internal hard drive using a program on the hard drive itself. You need to start up from your OS X installation disk or the first disk of the Software Recovery set that came with the computer. Failing that, you need to boot from an external hard drive/flash drive on which you installed OS X.

This is a screenshot from my MacBook running 10.4 Tiger from an external hard drive.

#1, Boot up from your OS X installation disk, the first disk of your Software Recovery disk set, or an external drive with OS X on it.

#2, Start up Disk Utility.

#3, Select the hard drive to erase from the list on the left hand side.

#4, You have four buttons next to each other on the Disk Utility screen. They are (1) First Aid (2) Erase (3) RAID (4) Restore

#5, You want to choose the Erase button. Click on it and it will turn blue with black letters.

#6, Now note Security Options… on the bottom right-hand side of the screen. Click on it to select how much security you want.

#7, Disk Utility offers you 4 levels of security. To achieve Department of Defense levels of security, choose the 7-Pass Erase

#8, If you’re really worried about the NSA or East European hackers getting your data, choose the 35-Pass Erase. This will take several days to accomplish. Set the computer up in a cool area with a fan blowing air across it.

#9, Having selected your level of security, click OK

#10, Now click Erase. Let the computer do its work and go about your life.

#11, The computer’s hard drive is erased. Dispose of computer at a proper recycling center. Do Not Put It in the Dumpster along with the fast food wrappers!

Here are some other articles on securely erasing a hard drive.

Christopher Breen of Macworld always has good ideas.

MakeUseOf has suggestions for securely wiping hard drives for all versions of OS X

Tom Briant

Editor, MacValley Blog