Going back to macOS

If Linux turns out not to be for you — on day three or two years in — here's the recipe to restore macOS exactly the way it was. It takes about half a day and it works.

Before you start

You need the Time Machine backup from backup & prep on an external drive.
If you have important files in Linux that aren't already in the backup (you've been using the machine for a while), copy them off first — to a USB stick, to another computer, to a cloud service. macOS won't see your Linux disk after the restore.
Plug the Mac into power. The restore will run for hours; you don't want it dying half-way.

The steps

Shut the Mac down completely. Not restart — full shutdown.
Boot into Recovery. Power on while holding one of these key combinations:
- ⌘-R (Command-R) — boots the Recovery partition that lives on the disk. Works if it survived your Linux install. Try this first.
- Option-⌘-R — Internet Recovery. Downloads a recovery image from Apple. Use this if the local Recovery is gone, or if you've replaced the disk entirely. Takes 5–30 minutes to download depending on your connection.
A spinning globe means you're in Internet Recovery; the macOS logo means you're in the local one. Either is fine.
Choose Disk Utility from the Recovery menu. Select your internal disk (the top-level one, not a partition). Click Erase. Format: APFS (the default on modern macOS). Name it whatever you like — "Macintosh HD" is the convention. Click Erase, wait a minute, quit Disk Utility.
Choose Reinstall macOS from the Recovery menu. Follow the prompts; it'll download the installer and write a fresh macOS to the disk. This takes 20–60 minutes.
Boot into the new macOS. Setup Assistant runs, like the first time you ever turned the Mac on. Pick your region, language, Wi-Fi.
When Setup Assistant asks "Transfer information to this Mac?", choose From a Mac, Time Machine backup, or startup disk. Plug in your Time Machine drive. Pick the most recent snapshot. Tick everything (apps, files, accounts, settings). Click Continue.
Wait. Two to six hours, depending on backup size and disk speed. Get coffee.
Sign into iCloud when prompted. Everything synced will reappear over the next hour or so.

You're back. The Mac is exactly as it was the night before you installed Linux.

On T2 Macs

If you lowered Secure Boot in Startup Security Utility during install, you can leave it lowered — or restore it to "Full Security". Either works for macOS. Restoring it makes the machine behave the way it shipped.

Activation Lock prompt?

On T2 Macs, after the restore the firmware may ask for the Apple ID password that was last signed in. This is Activation Lock — the same protection that's on iPhones. Sign in with the same Apple ID you used originally; it unlocks immediately. If you've forgotten that Apple ID, recover it through appleid.apple.com before continuing. (This is exactly why we sign out of Apple ID in step 2 of pre-flight before installing.)

What if you didn't take a backup?

You can still get back to a working macOS — you just lose your old data. Skip the Time Machine step; after the macOS reinstall, set up the Mac as new. Sign into iCloud and anything that was synced (Mail, Calendar, Contacts, Photos, Notes if you used iCloud) will come back from Apple's servers. Anything that was only local is gone.

Hopefully you read this page before installing, not after.

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