Clean install macOS

For most purposes, a standard upgrade of macOS works fine. If a Mac gives you trouble, see Fixing startup issues & crashes. If this doesn’t help you might choose to reload software on the computer.

Clean install

A clean install sets aside the old software so you can load the OS and apps from scratch rather than updating the previous (and potentially problematic) installation. You’ll erase the drive, so back up all data first.

  1. Back up the computer with Time Machine or Carbon Copy Cloner. Make sure all user files are present and up-to-date. Carbon Copy Cloner can often clone a non-working computer if you can boot from another drive.
  2. Restart the computer while holding Command-Option-R. Or start from a boot drive – hold the option key when turning on the Mac.
  3. Open Disk Utility and reformat the drive.
  4. Install macOS. See installing a fresh copy of macOS (bottom of the page).
  5. When the computer starts for the first time, choose the option to transfer files from a backup.
  6. Connect the backup drive and restore user folders. We do not recommend restoring apps or settings if you were having problems with them. See details.
  7. Reinstall software and settings. See Configuring settings and reloading apps above.

Optional method keeping user data in-place

WARNING: This is a technical process that should only be attempted by experienced techs. It preserves all user files, so it’s faster than reformating a drive and restoring user data. Even so, we highly recommend backing up first. Make sure nothing important has been excluded from backups.

Getting prepared

If you can, boot from the troubled Mac’s own drive for these steps. If that’s impossible, gather information from backups or users later in the process.

  1. Back up the computer with Time Machine or Carbon Copy Cloner.
  2. You’ll need approximately 25 GB of free space on the computer’s boot drive. 
  3. Make a note of the computer name in System Preferences – Sharing.
  4. Check the App Store and verify the Apple ID & password used for purchases.
  5. Note all user account (short) names & passwords.

Setting aside old software

We’re going to hide the OS & applications, while leaving user folders untouched. You’ll need to boot from another drive.

  1. If you’re using a boot drive, connect it to the Mac and restart, holding the Option key to select the boot drive.
  2. Or to use another Mac for preparation, connect the troubled computer with a Thunderbolt and restart it while holding the T key to put it in target disk mode. The computer appears on the other Mac as an external hard drive.
  3. Enable the root user & log in as root.
  4. Open Disk Utility and run First Aid on the drive. If the drive is OK or repairs successfully, go to step 4.
  5. If the drive cannot be repaired, clone the drive to a blank volume with Carbon Copy Cloner, then reformat. If there are hardware errors, have the drive replaced. After formatting, skip to Installing a fresh copy of OS X below.
  6. Click the Finder and type Shift-Command-. (period) to show invisible files.
  7. Create new folder at root level of the drive and name it Previous System.
  8. Select all items (including invisible items) at the root level of the drive – EXCEPT the following:
  • .DS_Store
  • .Trashes
  • Previous System
  • Users

Drag the selected items to the Previous System folder. Only the items listed above should remain.

Now install macOS. See installing a fresh copy of macOS (bottom of the page).

Restoring user accounts

After the reinstall, the computer should boot up normally and user files are present, but they are inaccessible until configured.

  1. The Setup Assistant opens, allowing you to create the first user account.
  2. Create a user account called Temp and create a password.
  3. Log in as the temp user.
  4. Open the Macintosh HD and Previous System. Open the Users folder and drag any users from here to Macintosh HD/Users. Make a note of the exact spelling of each user folder.
  5. Go to System Preferences. Click Users & Groups and create a new user. The account name must exactly match the name of the first user. 
  6. The system should offer to Use Existing Folder. If not, cancel and check the spelling.
  7. Create any other user accounts in System Preferences – Users & Groups.
  8. Carefully delete any accounts you may have made accidentally. When in doubt, delete the user but don’t change the home folder.

Configuring settings and reloading apps 

  1. Set the startup disk in System Preferences.
  2. Name the computer exactly as before in Sharing. One way to determine the old name is by examining the Time Machine backup folder.
  3. Set up network settings, printers & scanners, and other options in System Preferences.
  4. Configure Time Machine to back up to the drive that was used previously. You can exclude the Previous System folder.
  5. Reinstall software. Log into the App Store with the same Apple ID used for previous purchases.
  6. Log out, then log into each user account.
  7. Look for dock items, sidebar favorites & aliases that point to apps in the Previous System. These should be removed & recreated from items in the new Applications folder.
  8. After a period of real-world testing, drag the Previous System to the trash and empty it to regain space.

If you encounter serious user-specific problems or lost data, copy files from the Time Machine backup or clone of the original drive. Or use Migration Assistant to restore an entire user account from a backup or clone.

Learn more troubleshooting techniques.

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