Here are instructions on how to use Disk Utility to securely erase a hard drive connected internally or externally to your Mac. These instructions were written for Mac OS X 10.4 to 10.6. The process is very similar on older versions of Mac OS X but the screens will look a bit different.
Update Aug 2013: Newer Macs like the MacBook Air, Retina Display MacBook Pros and 2013 iMacs have solid state drives (SSD) drives which are also called flash storage. These drives can’t be securely erased using the process described below. Instead, use this clever method to securely erase your SSD or flash storage. This clever method involves enabling FileVault encryption on your drive and then erasing the drive. This BackBlaze article points out a way to securely erase an SSD using the Terminal application.
Update Nov 2014: If your Mac has OS X 10.7 or newer installed then Apple didn’t give you an OS X install disc with your Mac. Instead, you can startup your Mac from its Recovery Partition and then use its copy of Disk Utility to secure erase your hard drive. I describe how to do in a Tech Tip titled, Use The Recovery Partition to Securely Erase Your Mac’s Hard Drive. However, if your Mac has an solid state drive (SSD) then use the information listed in the paragraph above.
You’ll need either the Mac OS X install disk that came with your Mac or another OS X install disk that’ll boot your Mac, such as a retail copy of the Mac OS X 10.5 or 10.6 install disk.
I recommend securely erasing a hard drive before you recycle it or give it away. The instructions below include the option to re-install Mac OS X onto the hard drive at the end of the process.
If you’re securely erasing an external hard drive you do not need to use a Mac OS X install disk. Instead, you can simply open Disk Utility which you’ll find in the Utilities folder inside the Applications folder. In Disk Utility, select the external hard drive and then follow the last few steps of these instructions to erase it using one of the Security Options described below.
Technically, it’s not possible to erase or remove data from hard drives. Thus “securely erasing” is a misnomer. Instead, what we’ll be doing is over-writing the data on your hard drive with gibberish, a random string of bits of data. When you securely erase your hard drive you’ll have a choice of how much over-writing you want to do. This process can take many hours if you select the 7-pass overwrite procedure. I recommend starting this process and then letting it run overnight.
- Insert the Mac OS X install disc that came with your Mac.
- Note: The disc will begin the process of installing Mac OS X on your Mac. Whether or not you actually choose to install Mac OS X is your choice, but if you do, you’ll do it AFTER you’ve securely erased your Mac’s hard drive.
- Select your preferred language from the list presented.
- When you see the welcome screen for the Mac OS X Installer, go to the menu named Utilities and select Disk Utility.
- In the left pane of the Disk Utility window, click on the hard drive you want to erase.
- In the right pane of the Disk Utility window, click the Erase tab.
- Click the Security Options button.
- I recommend that you select the 7-Pass Erase options, but you could choose the 35-Pass Erase option. Here’s my description of these options.
Don’t Erase Data— This option does not provide any security. This option only over-writes the directories on the hard drive. Your files could still be recovered.
Zero Out Data— Provides a pretty good level of security. All of your files are over-written with a string of zeroes. There are forensics utilities that in theory could retrieve some data.
7-Pass Erase— Provides a lot of security. This is considered sufficient by government standards to erase data from a disk. It writes a random string of ones and zeroes over your data files seven times. It may take several hours or more to complete this process.
35-Pass Erase— Provides the maximum amount of security offered. This option takes an extremely long time, possibly more than one day.
- Click the radio button in front of 7-Pass Erase.
- Click OK.
- Click the Erase button next to the Security Options button.
- Confirm you want to erase and the process will be begin.
- If you want to install Mac OS X then go to the Disk Utility menu and select Quit after the erase is complete.
- You’ll be taken back to the Mac OS X Installer welcome window. Click Continue.
- Agree to the software license.
- In the “Select a Destination” window click on the icon for the hard drive which you just erased and then click Continue.
- Click the Install button to start the installation process. Depending upon the speed of your Mac and the version of Mac OS X which you are installing this process could commonly take anywhere from 20-60 minutes.
Update Oct 2014: In OS X 10.9, Mavericks, the 35-pass erase is no longer an option. Also, there are no longer radio buttons to select how many passes to erase. There is a slider that goes from Fastest to Most Secure.