How to securely erase your mac hard drive before selling it.

Securely erasing your hard drive before selling your mac.Macs hold their value very well, and it often makes sense to sell yours to help pay for the new model you are lusting after. Or you may be feeling generous, and you want to hand it down to someone needy. One question comes to mind. How do you secure your personal information? You don’t want the new owner to have access to your banking information and your chat logs, now do you? You really don’t want to just hand it over as is. While an OS reinstall should make your files inaccessible on the surface. However, if you Mac gets into the wrong hands, it’s still possible to dig deeper and recover deleted data. This MacTip will guide you through the Secure Erasing process, so your personal data won’t be recoverable no matter where your old Mac ends up.

Update: Since this article was written in 2011, Apple has stopped distributing OS X on DVD disks, and most of the computer lineup has forgone the optical drive altogether. In light of these changes, I’ve written an updated section for erasing and re-installing OS X using Recovery partition.

This guide will walk you though erasing your disk, and re-installing a fresh copy of OS X. Two methods depending on your situation:

1) With install DVD disk, versions OS X 10.6 or older.

2) No install disk, using OS X Recovery partition, for versions 10.7 (Lion) and newer.

Why you need to securely erase your hard drive

When you empty your trash bin on your Mac, the files are not really erased. Instead, the reference to the file is removed from the file system (which keeps a list of all the files on your computer), so the computer is able to write new information in the same physical area when space is needed. This is much quicker than rewriting that disk space with blank data, and it also gives you a chance to recover accidentally deleted files with handy recovery software. Erasing securely, or “shredding files” is the process of rewriting the disk with zeroes, or blank data so the files are impossible to recover.

With OS X Install Disk, versions OS X 10.6 or older

You will need:

  • A Mac OS X Install DVD that came in the box with your Mac, or a retail copy of OS X.
  • Some time. It may take a few hours to securely erase a hard drive, and another 30-60 minutes to install OS X.

How securely erase your data and reinstall Snow Leopard.

Steps may vary with different version of OS X, and hardware configurations but the idea is the same. You can’t securely erase the whole disk while it’s being used by the operating system, so you will have to boot Disk Utility from the Mac OS X Install DVD.

  1. We’re assuming you already copied all your data to an external backup, or to your new Mac, because the following steps will really erase your disk.
  2. Insert your Mac OS X DVD into your Mac, and restart the computer.
  3. After you hear the start up chime, hold down the Option key. You will be presented with a choice of booting from your hard disk, or the Mac OS X Install DVD. Select the DVD.
  4. Once the installer loads, and you get to the Install Mac OS X screen, instead of continuing, go to the Utilities menu on the top of your screen, and open Disk Utility.
  5. In Disk Utility, select your hard disk from the list on the left.
  6. Click the Erase tab in Disk Utility, select Mac OS Extended (Journaled) for Format.
  7. You will see a button near the bottom “Security Options”, this option wipes the drives clean of your old files.
  8. When you click on Security Options you will be presented with 3 levels of secure erasing. Pick one you are comfortable with.*Select the secure erase level
  9. Once the secure erasing process is finished, you may close Disk Utility and follow the prompts for a usual operating system installation.

No install disk, using OS X Recovery partition, OS X 10.7 or newer.

With OS X 10.7 Apple stopped distributing the operating system on optical disks (CD/DVD), in favor of digital distribution via the Mac App Store.

This method requires:

  • Your Mac had OS X 10.7 (Lion) or 10.8 (Mountain Lion) when it was purchased, you can do a fresh install of the same version.
  • If you upgraded your Mac to 10.7 or 10.8 via the Mac App Store, you will need your Apple ID login that was used to purchase the upgrade.
  • You will also need Internet connection (wi-fi or wired) to download the operating system from Apple. Preferably a fast one, as the download is larger than 4GB.

Please read these steps before starting, to understand the process and the requirements.

  1. Make sure that you made backup copies of the files. You’re about to format the drive, you won’t get them back.
  2. Restart the computer, and hold down the “Option” key when the screen turns white. If your computer is has a Recovery partition, you will see an icon for your hard drive, and another for Recovery. Select the recovery partition to boot into it.
  3. Select your language.
  4. When the you are presented with the OS X Utilities menu, we first need to launch the Disk Utility to erase the contents from the disk.
  5. In Disk Utility, Select your Mac’s disk in the upper left hand corner and view the Erase tab. From the drop down box select Mac OS X Exteneded (Journaled) as the format, and give the disk a name.
  6. If your Mac is equipped with a Solid State Disk (SSD), you will notice that the “Security Options” button will be greyed out and unavailable. SSDs store data in a different way from traditional hard drive. Apple disabled Security Options for SSDs because they claim that a standard erase will makes data recovery difficult. If however, the Security Options button is availible, go ahead and use it, but it will take a few hours depending on the size of the hard drive and the number of times you want it overwritten.
  7. Once the Disk Utility is finished formatting the drive, close it to return to OS X Utilities.
  8. Now we are ready to Reinstall OS X.
  9. Read and accept the license agreement, and when prompted for an install loaction, select the disk you just erased.
  10. At this point:
    • If you are restoring the same version that came with your Mac, the download will start right away.
    • If you upgraded to a newer version of OS X and you upgraded to a newer version through the Mac App Store. You will be prompted for your Apple ID before the download starts. Enter the same ID you used to purchase the OS X upgrade.
  11. After you are authorized, the computer will begin downloading the operating system from the Internet. The download may take some time.
  12. After the download is finished, the computer will begin to install the operating system. This may take 20 to 40 minutes.
  13. After the installation process is completed, the computer will restart and greet you with a welcome message.

    You can choose to enter in the setup information and create a new account, or power off the computer and leave it up to the new user.