How to Do a Clean Install of macOS 27

If you prefer a cleaner installation of macOS 27 Golden Gate than what an in-place upgrade provides—specifically, one that gets rid of any disk errors and gives you a chance to weed out some types of user-created data—opt for Plan B. It’s also the right choice if you tried Plan A and encountered problems.

Much of Plan B is just like Plan A. The differences come at the beginning (erasing your disk manually) and the end (using Setup Assistant to configure numerous settings and, optionally, using Migration Assistant to transfer data from your backup).

Start up from another volume

Unlike Plan A’s in-place upgrade, Plan B requires you to start up from another volume that contains the macOS 27 installer. You have a few choices:

• A bootable installer volume (see how to make one)
• A bootable duplicate stored on an external SSD
• Recovery mode

The first two are covered next; recovery mode follows after.

Start up from an external drive

To start up from a bootable installer volume or a bootable duplicate:

1. Make sure the external drive is connected to your Mac.
2. Choose Apple menu > Shut Down.
3. Press and hold the power button until the text “Loading startup options” appears.
4. Wait for the available startup volumes to appear; this may take 30 seconds or more.
5. Select your external startup volume and click Continue.

Now that you’ve booted from another drive, you can erase your main disk and install macOS 27 onto it.

Start up in recovery mode

To start your Mac in recovery mode, turn it off, then press and hold the power button until you see “Loading startup options” on the screen. When the Options icon appears, click it, then click Continue.

Erase your disk

Having started up from another volume or in recovery mode, follow these steps to erase your destination volume in preparation for macOS 27:

1. Open Disk Utility. If you’ve started up from an external volume, you can find it in /Applications/Utilities. If you’re in recovery mode, click Disk Utility in the macOS Utilities window.
2. If only the volumes are visible in the sidebar and not the drive itself, choose View > Show All Devices.
3. In the sidebar, select the disk or partition you want to erase. (Be sure to select the top-level icon, not an indented container or volume.) If it is not already mounted, click the Mount icon on the toolbar; if prompted to unlock it, enter your password.
4. On the toolbar, click the Erase button.
5. Type a name for your disk, make sure Format is set to APFS, and if the Scheme pop-up menu appears, choose GUID Partition Map.
6. Click Erase.
7. If you see a confirmation dialog, click Erase again. Disk Utility erases the disk, which normally takes just a few seconds. Click Done if prompted when the Erase process is complete.
8. Choose Disk Utility > Quit Disk Utility.

Triple-check to be sure that the volume you’re erasing is the one where you want to install macOS 27, and that it’s the one you’ve backed up following the backup instructions.

You’re now ready to install macOS 27 on the empty volume.

Run the installer

Your next steps are slightly different depending on how you started your Mac:

• If you’ve started from an external drive, make sure you’ve quit all running apps and then double-click the Install macOS Golden Gate icon in your /Applications folder or wherever else you put it. (If you haven’t quit all running apps, the installer will attempt to do so.)
• If you’re running in recovery mode, click Install macOS (or Install macOS Golden Gate).

A window appears with one button: Continue. Click it!

The next few steps are identical to those for an in-place upgrade:

1. The Software License Agreement appears; click Agree, and then click Agree again in the confirmation dialog.
2. Now select your destination. If needed, click Show All Disks to display all available options and click the one you want to use.
3. With your desired destination volume selected, click Continue or Install (the wording varies depending on your setup).
4. Next, you’re prompted to enter your administrator credentials (in order to add a helper app); do so and click OK (or, in some cases, Unlock).

The installer begins copying files, a process that typically takes just a few minutes, but may take longer. Then, depending on your configuration, it may restart your Mac automatically, or it may ask you to click a Restart button. After the restart, the main portion of the installation process occurs.

If you are installing macOS 27 onto a disk that’s encrypted, you may be prompted to enter your password to unlock the disk immediately after the installer restarts your Mac.

The installation itself takes as long as an in-place upgrade does. As the installer progresses, your Mac will restart one or more times, with or without the startup chime. You’ll know the process has finished when the Select Your Country or Region screen appears. That’s part of Setup Assistant.