Erase or Format Your Macs Drives Using Disk Utility

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Getting to Know Disk Utility

Disk Utility ready to use
The Disk Utility app contains a toolbar and sidebar for ease of use. Screen shot courtesy of Coyote Moon, Inc.

Disk Utility, a free application included with the Mac OS, is a multipurpose, easy-to-use tool for working with hard drives, SSDs, and disk images. Among other things, Disk Utility can erase, format, repair, and partition hard drives and SSDs, as well as create RAID arrays. In this guide, we'll use Disk Utility to erase a volume and format a hard drive.

Disk Utility works with disks and volumes. The term 'disk' refers to the drive itself; a 'volume' is a formatted section of a disk. Each disk has a minimum of one volume. You can use Disk Utility to create a single volume or multiple volumes on a disk.

It's important to understand the relationship between a disk and its volumes. You can erase a volume without affecting the rest of the disk, but if you erase the disk, then you erase every volume that it contains.

Disk Utility in OS X El Capitan and Later

Disk Utility underwent some changes in the version included with OS X El Capitan, as well as the new macOS version of the operating system. This guide is for the version of Disk Utility found in OS X Yosemite and earlier.

If you need to format a drive using OS X 10.11 (El Capitan) or later, check out:

Format a Mac's Drive Using Disk Utility (OS X El Capitan or later)

Let's Get Started

Disk Utility has three main sections: a toolbar that spans the top of the Disk Utility workspace; a vertical pane on the left that displays disks and volumes; and a work area on the right, where you can perform tasks on a selected disk or volume.

  • Launch Disk Utility, located at /Applications/Utilities/.

Since you will use Disk Utility for system maintenance purposes as well as for working with hard drives, I recommend adding it to the Dock. Right-click the 'Disk Utility' icon in the Dock, and select 'Keep in Dock' from the pop-up menu.

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Disk Utility: Erasing a Non-Startup Volume

Disk Utility Erase
Disk Utility can quickly erase a volume with just a click of a button. Screen shot courtesy of Coyote Moon, Inc.

Erasing a volume is an easy way to free up drive space. Many multimedia applications, such as Adobe Photoshop, need a large amount of contiguous disk space to work in. Erasing a volume is a faster way of creating that space than using third-party defragmenting tools. Because this process erases all of the data on a volume, many multimedia-savvy individuals create small volumes to hold a project's worth of data, and then erase the volume before starting the next project.

The data erase method outlined below doesn't address any security issues that may be associated with the erased data. In fact, most data recovery programs would be able to resurrect the data that was erased using this simple process. If you're concerned about security, consider using the secure erase procedure addressed later in this guide.

Erase a Volume

  1. Select a volume from the disks and volumes listed in the left side of the Disk Utility window. Each disk and volume will be identified by the same name and icon that it displays on the Mac desktop.
  2. Click the ‘Erase’ tab. The selected volume's name and current format will display in the right side of the Disk Utility workspace.
  3. Click the ‘Erase’ button. Disk Utility will unmount the volume from the desktop, erase it, and then remount it on the desktop.
  4. The erased volume will retain the same name and format type as the original. If you need to change the format type, see How to Format a Mac's Hard Drive Using Disk Utility, later in this guide.
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Disk Utility: Secure Erase

Disk Utilities Secure Erase Options
Use the slider to select one of the secure erase options. Screen shot courtesy of Coyote Moon, Inc.

Disk Utility provides four options for securely erasing the data on a volume. The options include a very basic erase method, a slightly more secure erase method, and two erase methods that meet or exceed U.S. Department of Defense requirements for erasing confidential data from hard drives.

If you're concerned about someone being able to recover the data you're about to erase, use the secure erase method outlined below.

Secure Erase

Select a volume from the disks and volumes listed in the left side of the Disk Utility window. Each disk and volume will be identified by the same name and icon that it displays on the Mac desktop.

Click the ‘Erase’ tab. The selected volume's name and current format will display in the right side of the Disk Utility workspace.

Click the ‘Security Options’ button. A Security Options sheet will display the following secure erase options depending on the version of the Mac OS you are using.

For OS X Snow Leopard and Earlier​

  • Don’t Erase Data. This method is quick, but not very secure. It removes the volume's catalog directory, but leaves the actual data intact.
  • Zero Out Data. This method provides a good level of security. It erases the volume's data by writing over it with zeros. The length of time this method takes depends on the size of the volume.
  • 7-Pass Erase. This method meets the security requirements of the U.S. Department of Defense's 5220-22M standards for erasing magnetic media. This erase method can take a very long time.
  • 35-Pass Erase. This is the highest level of data erase security that Disk Utility supports. It writes over the data on a volume 35 times. Don't expect this erase method to finish any time soon.

For OS X Lion Through OS X Yosemite

The dropdown Secure Erase Options sheet presents options similar to the ones available in earlier versions of the operating system, but it now uses a slider for making choices instead of an options list. The slider options are:

  • Fastest. This is the quickest erase method. It doesn't scramble the file data, which means a recovery app may be able to resurrect the erased data.
  • Zero Out the Data. This erase method writes a single path of zeros to all locations on the selected volume or disk. Advanced recovery techniques could restore the data, but it would require a great deal of time and effort.
  • Three-Pass. This is a DOE compliant three-pass secure erase. It writes two passes of random data to the volume or disk, and then writes a single pass of a known data pattern to the volume or disk.
  • Most Secure. This method of securely erasing a volume or disk meets the requirements of the U.S. Department of Defense (DOD) 5220-22M standard for securely erasing magnetic media. The erased volume is written to seven times to ensure the data can't be restored.

Make your selection and click the ‘OK’ button. The Security Options sheet will disappear.

Click the ‘Erase’ button. Disk Utility will unmount the volume from the desktop, erase it, and then remount it on the desktop.

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How to Format a Mac's Hard Drive Using Disk Utility

Formatting options available in Disk Utility
Use the drop down menu to select formatting options. Screen shot courtesy of Coyote Moon, Inc.

Formatting a drive is conceptually the same as erasing it. The main difference is that you will select a drive, not a volume, from the list of devices. You will also select the type of drive format to use. If you use the formatting method that I recommend, the formatting process will take a little longer than the basic erase method described earlier.

Format a Hard Drive

  1. Select a drive from the list of drives and volumes. Each drive in the list will display its capacity, manufacturer, and product name, such as 232.9 GB WDC WD2500JS-40NGB2.
  2. Click the ‘Erase’ tab.
  3. Enter a name for the drive. The default name is Untitled. The drive's name will eventually appear on the desktop, so it's a good idea to choose something that's descriptive, or at least more interesting than "Untitled."
  4. Select a volume format to use. The Volume Format dropdown menu lists the available drive formats that the Mac supports. The format type that I recommend using is ‘Mac OS Extended (Journaled).'
  5. Click the ‘Security Options’ button. A Security Options sheet will display multiple secure erase options.
  6. (Optional) Select ‘Zero Out Data’. This option is for hard drives only, and should not be used with SSDs. Zero Out Data will perform a test on the hard drive as it writes zeros to the drive's platters. During the test, Disk Utility will map out any bad sections it finds on the drive's platters so they can't be used. This helps ensure that you won't be able to store any important data on a questionable section of the hard drive. This erase process can take a fair amount of time, depending on the drive's capacity.
  7. Make your selection and click the ‘OK’ button. The Security Options sheet will disappear.
  8. Click the ‘Erase’ button. Disk Utility will unmount the volume from the desktop, erase it, and then remount it on the desktop.
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Erasing or Formatting a Mac's Startup Drive Using Disk Utility

Recovery Utility
OS X Utilities is part of the Recovery HD, and includes Disk Utilities. Screen shot courtesy of Coyote Moon, Inc.

Disk Utility can't directly erase or format a startup disk, because Disk Utility, and all of the system functions it uses, is located on that disk. If Disk Utility tried to erase the startup disk, it would at some point erase itself, which could present a bit of a problem.

To get around this problem, use Disk Utility from a source other than the startup disk. One option is your OS X Install DVD, which includes Disk Utility.

Using Your OS X Install DVD

  1. Insert the OS X Install DVD in your Mac's SuperDrive (the CD/DVD reader).
  2. Restart your Mac by selecting the 'Restart' option in the Apple menu. When the display goes blank, press and hold the 'c' key on the keyboard.
  3. Booting from the DVD can take a little time. Once you see the grey screen with the Apple logo in the middle, you can release the ‘c’ key.
  4. Select ‘Use English for the main language.' when this option appears, then click the arrow button.
  5. Select 'Disk Utility' from the Utilities menu.
  6. When Disk Utility launches, follow the steps outlined in the 'Erase a Non-Startup Volume' section of this guide.

Using the OS X Recovery HD

  1. For Macs that do not have an optical drive, you can boot from the Recovery HD to run Disk Utility. Starting Up From the OS X Recovery HD Volume
  2. You can then use the steps found in the Erase a Non-Startup Volume on page 2.

Restart Your Mac

  1. Quit Disk Utility by selecting 'Quit Disk Utility' from the Disk Utility menu item. This will take you back to the Install OS X window.
  2. Quit the OS X Installer by selecting 'Quit OS X Installer' from the Mac OS X Installer menu item.
  3. Set the startup disk by clicking the ‘Startup Disk’ button.
  4. Select the disk you want to be the startup disk and then click the ‘Restart’ button.