How to Wipe a Mac and Reinstall macOS

Introduction

This article provides step-by-step instructions for backing up and securely wiping a Mac, and then installing a fresh version of macOS.  This is useful if the Mac is being given to another user, or if the machine is not performing well and a clean reinstall would be helpful.  

 

Procedure

 

Backup Your Data Using Time Machine

1. Have an external hard drive with enough storage capacity to backup your entire system. 

(If in doubt as to what size external hard drive to use, go to the Apple icon in the top left of the screen and click "About My Mac", then click the "Storage" tab to see how much data is currently stored on your Mac's hard drive)

1. Connect the external hard drive to the Mac. 
2. Launch Disk Utility.  Applications > Utilities > Disk Utility

 

3. Select the external hard drive from the drive list on the left and click "Erase".

 

4. From the drop-down menu, choose “Mac OS Extended (Journaled)” as the "Format" type.  You don't need to fill out the "Name" section for now, and then click "Erase".

 

5. When the drive has finished formatting, choose the "Partition" tab. 

 

6. In the Partition Layout menu, click on the “+” sign once to split the drive into two equal partition sizes.

 

7. (Optional) Adjust the partition size by dragging the small white dot located on the perimeter of the large blue circle, or manually, by clicking on a partition and then entering the desired partition size in the "Size" input box. 

 

8. Name each of the two partitions by clicking on the first partition half and naming it something like "Time Machine Backup", then select the other partition half to name it something like "File Storage"(Step 3). Finally, click “Apply” (Step 4). 

 

9. Confirm the changes by clicking "Partition".

 

10. Back up the Mac to the Time Machine Backup partition by going to System Preferences in the Apple menu, then choose "Time Machine". 

 

11. Click "Select Backup Disk"  to let the list populate. Check “Show Time Machine in the menu bar” so that you can monitor its progress easily.

 

12. Choose the partition named "Time Machine Backup" from the list and then confirm the choice by clicking "Use Disk". 

 

13. Let Time Machine back up for the first time. (Note this may take some time depending on how much data there is on the internal drive). 
14. Once the backup is done, Time Machine will provide the following confirmation:

 

Reinstall New Operating System

1. If you are setting this machine up for a new user, sign out of all applications on the Mac.  If you are setting this up for the same user, disregard these steps.

   1. Deauthorize and sign out of iTunes

      1. Open iTunes from the applications menu. Finder > Applications > iTunes.

 

2. Choose Account > Authorizations > Deauthorize This Computer.  When prompted, enter your Apple ID and password, then click "Deauthorize".

 

3. Click “Account” in the menu bar on the left side of the screen and click “Sign Out”. Finally, click iTunes > Quit iTunes.

 

2. Sign out of iMessage

   1. Open Messages on your Mac. Finder > Applications > Messages.

   2. Click “Messages” in the menu bar at the top left corner of your screen (Step 1). 

   3. Click “Preferences” from the drop down menu (Step 2). 

   4. Select “iMessages” account (Step 3) and click “Sign Out” (Step 4).

 

3. Sign out of iCloud

   1. Go to the Apple icon in the top left corner of your Mac screen. Select System Preferences from the drop down menu. Click on iCloud. 

 

2. Uncheck the box for "Find my Mac". 

3. Click “Sign Out”.

4. A dialog will ask whether you want to keep a copy of your iCloud data on the Mac.  Because you’ll reformat the hard drive in a later step, just click “Keep a Copy” to proceed.  After you’ve signed out of iCloud, the user’s data remains in iCloud and on any of their other devices. 

 

 

2. Erase the hard drive and reinstall macOS

Once the backup and unlinking of accounts is completed, you can erase and reformat the drive.

1. Restart the machine and hold down CMD+R as soon as the restart begins. 
2. Select "Disk Utility" then click "Continue".

 

3. Choose View > Show All Devices from the menu bar

1. On the left pane, select the volume containing the operating system (typically called Macintosh HD), select the "Erase" tab and enter the following information:

• Name: Enter a name for the new volume, such as “MacIntosh HD”
• Format: Choose “Mac OS Extended (Journaled)
• Scheme (if shown): Choose "GUID Partition Map"

2. Click "Erase".

 

4. Quit Disk Utility when done and then select "Reinstall OS X" and click "Continue".  Reinstallation will take about an hour.

5. Have the client sign in with their Apple credentials in order to reinstall their apps.