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Cybersecurity at NC State

Erasing Hard Disks for Macs

To erase a Macintosh hard disk, you can boot with a different disk and use the Disk Utility.

OS X Versions 10.2 through 10.7

NOTE: This refers to a full Mac OS X 10.2.3 or later CD included with a computer, not to the update-only 10.2.3 CD. 

Zeroing Data

The following procedure assumes you have a Mac OS X 10.2.3 or later CD:

  1. Insert the Mac OS X CD.
  2. Restart the computer.
  3. Immediately after the startup sound, press and hold the “C” key to start up from CD.
  4. When the Installer screen appears, do not click Continue. Instead, choose Installer > Open Disk Utilities.
  5. Select the hard drive you want to erase.
  6. Click the Erase tab.
  7. Select the volume format you want from the Volume Format pop-up menu.
  8. Click Options.
  9. Select the Zero all data checkbox.
    1. NOTE: For a more secure method of erasing of data, you can select the 8 Way Random Write Format checkbox instead of or in addition to Zero all data. 
    2. This process could take several hours.
  10. Click OK.
  11. Click Erase.

OS Versions 10.8 and Newer

Secure Erase is documented in the Help menu of the Disk Utility application. Disk Utility is included for all OS X versions in the /Applications/Utilities folder.

To erase your computer’s startup disk, you must start up from another disk, such as an OS X installation disc or from the Recovery Disk.  All new OS devices have a built-in Recovery Disk that can be used by restarting and holding down the R key.  A Recovery Disk can also be created on an external hard disk or thumb drive using the Recovery Disk Assistant per the https://support.apple.com/en-us/HT202294 website.

Securely Erase a Hard Disk in OS X 10.8 or Newer

This section provides a summary of the Disk Utility help available from the Help menu when running the Disk Utility application.

You can securely erase a disk or partition to make sure the confidential information it contains cannot be recovered.

When erasing a disk or partition, Disk Utility does not erase the actual files; it erases only the information used to access the files. Many commonly available disk recovery apps can easily recover the files. When securely erasing a disk or partition, Disk Utility writes over the erased files, so they can never be recovered. You can choose how many times to write over the files. Writing over the files several times is more secure than writing over them once, but it takes longer.

Securely erasing a disk erases files on all its partitions. Securely erasing a partition erases all the files on that partition but does not affect other partitions.

You can’t securely erase a CD, DVD, or a solid-state drive. If you don’t want data recovered from a solid-state drive, use full disk encryption.

To erase your computer’s startup disk, you must start up from another disk, such as an OS X installation disc or from the Recovery Disk.

  1. Open the Disk Utility application from the Utility Folder in the Applications folder or select it from the list when booting from the Recovery Disk.
  2. Select the disk or partition to erase, and then click Erase.
  3. Specify a format, and then enter a name for the disk.
  4. Click Security Options, and then use the slider to choose how many times to write over the data on the disk.
  5. Click OK.
    1. NOTE: Writing over the data three times meets the U.S. Department of Energy standard for securely erasing magnetic media. Writing over the data seven times meets the U.S. Department of Defense 5220-22-M standard. Single pass or “Zeroing out” the date is adequate for most cases.
  6. Click Erase.

Optional Procedure to Clean Individual Files

If you want to “clean” individual files, note the following section from the technote:

Mac OS X 10.3+ feature — Secure Empty Trash

You can securely empty the Trash in Mac OS X Panther. While in the Finder, go to the File menu and select Secure Empty Trash.

CAUTION: When you securely empty the Trash, the deleted data cannot be recovered by disk utilities. Do not do this unless you have a backup or xare sure you will never need the data again.For machines that boot only MacOS 9 or earlier, use Drive Setup, Apple HD SC Setup, or Internal HD Format and select the option to zero out and/or low-level format the disk. These programs are usually in the Utilities Folder on the installation CDs for the various versions of the MacOS.