The macOS 27 Golden Gate installer doesn’t give you many options, but you can still approach the process in either of two ways:
- Plan A, an in-place upgrade: This easy, default method is appropriate for most people, with some qualifications.
- Plan B, a clean install: Although the installer offers no built-in option to start fresh by erasing your disk, you can accomplish the same thing in a slightly roundabout way.
Most users should start with Plan A and then if (and only if) they encounter problems, move on to Plan B. A few people may want to skip Plan A and go directly to Plan B. In almost every case, the end result will be virtually identical, regardless of which path you take.
Upgrading a new Mac
If you’re not only upgrading a new (or new to you) Mac to macOS 27 but also moving your files from an old Mac, you can use Migration Assistant to transfer your old data to the new Mac. If you use Plan B: Clean Install, Migration Assistant runs automatically at the end of the installation; if you use Plan A: In-Place Upgrade, you can run the Migration Assistant utility immediately after rebooting in macOS 27.
Apple has made a major improvement to Migration Assistant: if it sees an account on the old Mac with the same username as one on the new Mac, it offers to delete the user account from the new Mac and replace it with the data from the old Mac, optionally keeping an archive of the existing account.
Plan A: in-place upgrade
The macOS 27 installer offers a single upgrade method that attempts to make the transition simple by leaving almost all your files, apps, and settings in place. You run the installer as you would any other app, and it replaces all the components of your old macOS installation with their macOS 27 equivalents, while deleting those that are obsolete—just as the App Store app does when it installs minor updates.
The installer automatically checks for certain categories of incompatible software and moves any such files safely aside into a Relocated Items folder in /Users/Shared.
The installer can’t know about every potential software conflict with macOS 27—it disables only software that loads automatically on startup and that runs in the background, such as drivers and other kernel extensions. Ordinary apps are never moved aside, even if they can’t possibly run under macOS 27.
As long as you have a complete backup, you have little to fear from an in-place upgrade to macOS 27, and it’s entirely reasonable for Apple to have made it the only official option.
However, I must reiterate that as good as the installer is, there are no guarantees. Something could still go wrong before, during, or after the upgrade. It’s also possible that, due to an error or design flaw in the installer, important files could be deleted. These are among the reasons to consider following the clean install procedure.
Plan B: clean install
Whether or not you trust the installer (and macOS 27 itself) to protect you from software incompatibilities, you might consider making a fresh start by erasing your entire disk and installing a clean, pristine copy of macOS 27 Golden Gate. This process requires a few extra clicks and a bit more time, but it’s not hard to perform. You must start up from another volume (which could be a bootable duplicate or a bootable installer volume) to erase your previous system, run the installer, and restore your personal files.
The main reason to prefer a clean install over an in-place upgrade is that erasing your disk wipes out any hidden directory corruption, file system problems, and other disk gremlins that may cause problems.
If a clean install did nothing but erase your disk and install macOS 27, then all your settings, user-installed software, and files would be gone, and you’d have to either restore them from a backup or recreate them from scratch: not fun. But as in previous versions of macOS, the macOS 27 installer also offers an optional Migration Assistant feature—available only when you install onto a blank volume—that makes the process easy. As soon as the installation completes, you’ll have the option to migrate all your apps and personal files from another volume. Since you’ve made a complete backup, the installer can take all your personal data from that disk and copy it to the analogous location on the macOS 27 volume.
When copying your old data, you can be somewhat selective, but you can’t select individual files or folders to migrate (or skip). If you select every possible category of data (the default), Migration Assistant dutifully copies virtually everything from your backup to your startup volume (while moving aside any software it identifies as incompatible, just as occurs in an in-place upgrade). Almost to a file, the results should be identical to what you’d get with an in-place upgrade! (There are some tiny differences, but almost certainly nothing you’d ever notice if you didn’t scrutinize every last file on your disk.)
The thing is, I recommend that you do copy all your old data, because otherwise you’re likely to be left without something you need and expect. Therefore, since the net result is essentially the same with the in-place upgrade method and a clean install, I now recommend a clean install only in the following situations:
- Your startup disk has errors that Disk Utility can’t repair, even in recovery mode.
- You want to be selective about which folders and files you let Migration Assistant copy (to reduce the clutter on your disk, for example), and you’re willing to manually copy other files you may need later.
- You’re installing macOS 27 onto a Mac you just purchased, and you want to erase the drive and start fresh. You’ll transfer your data from your old Mac (or PC) during the installation process.
- You attempted an in-place upgrade and it failed.
I want to remind you that Plan B really will destroy all the data on your disk (or partition) prior to installing a fresh copy of macOS 27. If anything were to go wrong with your backup, you would be unhappy. So check your backups!
Pick a plan
To summarize, I believe Plan A is best for most people. Let Apple’s installer do its thing. You’ll already have a full backup (because, remember, I insist on it!), so the worst thing that could happen is you lose an hour or so of your time and have to start over with Plan B’s clean install. The overwhelming probability is that everything will be just fine, however, if you go with the flow.
With any plan—even with an in-place upgrade—questions, choices, and issues may arise. I explain how to deal with all of them as we go.
