Decide on an Installation Method

The High Sierra installer doesn’t give you many options, but you can still approach the process in either of two ways:

• Plan A: In-place Upgrade: This easy, default method is appropriate for most people (including anyone Upgrading from the High Sierra Beta), with some qualifications.
• Plan B: 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.

In the pages that follow, I describe the differences between these two plans and help you choose which way to go. Then, in later chapters, I give you step-by-step instructions for each plan.

Plan A: In-place Upgrade

As has been true since Snow Leopard, the High Sierra 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 High Sierra equivalents, while deleting those that are obsolete—just as the App Store app does when it installs minor updates.

The High Sierra installer even helps you avoid many common incompatibilities. It contains a list of software known to conflict with the new operating system, and when you run the installer, as long as you have an active internet connection, it even checks for additions to the list. Software that’s on the list is moved safely aside into an Incompatible Software folder.

In addition, some older software whose design is incompatible with High Sierra is left in place but rendered inert by the design of the operating system itself. This includes certain status menus, drivers, and other background processes. Still other incompatible items, such as older Apple Mail plug-ins, are disabled automatically when you first run the software that uses them.

As long as you have a complete backup, you have little to fear from an in-place upgrade to High Sierra, and it’s entirely reasonable for Apple to have made it the default—indeed, 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 entertain the possibility of following the clean install procedure that I describe next.

Plan B: Clean Install

Whether or not you trust the installer (and High Sierra itself) to protect you from software incompatibilities, you might consider making a fresh start, erasing your entire disk and installing a clean, pristine copy of High Sierra. This process requires a few extra clicks and a bit more time, but it’s not hard to perform—you merely have to start up from another volume (which could be your bootable duplicate) 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. As a bonus, your entire disk is automatically defragmented, because the new files from your High Sierra installation and your backup are written to it in contiguous chunks. This can, in some situations, improve disk performance (though only for hard disks, not for SSDs).

If a clean install did nothing but erase your disk and install High Sierra, then all your preferences, network 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 other recent versions of macOS, the High Sierra installer offers an optional File Transfer feature—available only when you install onto a blank volume—that makes the process easy. As soon as the installation completes, Setup Assistant should run automatically and offer to migrate all your apps and personal files from another volume. Since you’ve made a bootable backup onto another disk, the installer can take all of your personal data from that disk and copy it to the analogous location on the High Sierra volume.

When copying your old data, you can be somewhat selective—copy only the contents of your home folder, say, or your third-party apps, or any of various other combinations—but you can’t select individual files or folders to migrate (or skip). If you select every possible category of data (the default ), Setup 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 by performing an in-place upgrade! (There are some tiny differences, such as certain cache and log files, 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 the same with the in-place upgrade method and a clean install, I now recommend the latter only in the following situations:

• You want the benefits of a freshly optimized hard disk—or your disk (or SSD) has errors that neither Disk Utility nor third-party utilities can repair.
• You want to be selective about which folders and files you let Setup Assistant copy (to reduce the clutter on your startup volume, for example, or to have a cleaner setup for testing purposes), and you’re willing to manually copy other files you may need after the fact.

• You’re installing High Sierra onto a Mac that you just purchased, and you want to erase the drive and start fresh with High Sierra. You’ll transfer your data from your old Mac or Windows PC during the installation process.
• You attempted an in-place upgrade and it failed, or it left your Mac in an unusable state.
• You’re paranoid and you wouldn’t trust an in-place upgrade no matter what assurances I give you!

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 bootable duplicate (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’ll explain how to deal with all of them as we go.