We all hope that it’ll never happen, but even Macs sometimes break. This article explains what you should do – when possible – before your Mac goes for service or repair, so that when it returns you can get up and running as quickly as possible. Apple’s advice is brief, but makes the two most important points:
“Before you return your Mac to us, be sure to enable FileVault and back up your data. You may need the backup if your Mac needs to be reformatted during the repair process. Apple isn’t responsible for lost data.”

Preparation

If your Mac is going in for service or repair, be prepared for it to return with its internal storage wiped or replaced with new. Even if the repair shouldn’t result in logic board replacement, that may prove necessary, and in many cases that will bring with it a replacement SSD. If your Mac has a T2 chip, that will also necessarily be replaced with the whole logic board.

If your Mac has a severe fault, you’ll probably not have time to make special preparations. But if you can, it’s worth taking precautions before you take or send your Mac off as they will make life so much easier when it comes back. Ensure that you have a current Time Machine backup just before shutting it down, and/or make a bootable clone of its internal storage to an external drive.

If your Mac is anything like mine, and full of lots of documents which would take a long time to replace, you’ll surely want to do both. Time Machine backups are invaluable, but sometimes don’t work perfectly. A clone copy made using Carbon Copy Cloner or SuperDuper gives you added flexibility, and a second copy of everything on your Mac.

If you store data on your Mac’s internal storage which needs to be protected, you should also think carefully how best to ensure that it remains safe while your Mac is in the care of others. If the boot disk is already protected using FileVault, you shouldn’t need to do anything more. If your Mac has a T2 chip, turning FileVault on is instantly effective, as all the data stored on its boot disk is already encrypted; it’s still worth turning it on, though, as that requires your password before the encrypted files can be accessed.

If your Mac doesn’t have a T2 chip and FileVault isn’t turned on, encrypting your whole boot volume can take a very long time. Consider creating an encrypted sparsebundle, and moving the sensitive files to that. Don’t forget to delete the originals, though.

The best sequence to follow is this:

1. When you’re ready, close all applications including Mail and social network browsers, ready to shut your Mac down.
2. Make one last Time Machine backup, using the Back Up Now command in Time Machine’s menubar entry, and/or clone its internal storage to make a bootable backup on an external drive.
3. Turn FileVault on, encrypt or remove all sensitive files.
4. Disable any hardware password.
5. If necessary, turn off Find My Mac to disable it and Activation Lock, then sign it out of iCloud.
6. Shut your Mac down using the Apple menu command, if you can.

Pack your Mac carefully using its original packaging, if possible, and take or send it to Apple or Apple’s Authorised Service Provider. Unless specifically indicated or instructed, I generally avoid sending any cables, input devices, or anything else when the work is to be done on the Mac itself. That way, there’s nothing else to go missing.

To disable Activation Lock and sign out of iCloud in Catalina, open the Apple ID pane and select iCloud at the left. Uncheck the Find My Mac item in the main list, then select the Overview item at the left and click the button to Sign Out from iCloud.

You’ll almost certainly need to give the service provider the password which they’ll need to access your Mac through its FileVault protection, or they won’t be able to boot it up. Interestingly, Apple doesn’t tell its customers that will be required, but without it the service engineer will be unable to boot any Mac with a T2 chip and Secure Boot enabled. Experience is that service providers will require that password, or will refuse the job.

Return

When your Mac returns, connect it to a minimum of essential peripherals and try starting it up normally. If it goes straight into the macOS setup sequence, restart into Recovery Mode and open Disk Utility. If almost everything on your boot volume has vanished, you know that its internal storage has been wiped or replaced.

If your Mac has a T2 chip and you want to use a bootable clone to restore its contents, now is the time to open Startup Security Utility and enable it to boot from an external drive (see below).

If you need to restore your Mac’s internal storage, you now have a choice of routes for that journey, which include:

• If your Mac boots into setting up macOS, you can follow the setup, and opt to migrate from the external clone or your Time Machine backup.
• Whatever happens, you can choose a bootable external clone as your startup disk, and use Carbon Copy Cloner or SuperDuper to clone that back to your internal storage. Remember to use the Startup Security Utility in Recovery Mode to allow this for a Mac with a T2 chip, if you want that option.
• You can set macOS up as a new user, and migrate using Migration Assistant afterwards, or migrate manually if you really want to.

Whichever you choose, you’ll want to ensure that your Time Machine backup or clone is connected by the fastest possible method, such as Thunderbolt 3, to make the process as rapid as possible.

The only complication with a T2 chip is when it comes to getting it to boot from an external disk, if that is how you have decided to restore your internal storage when the Mac returns. So long as you’re using a current version of the cloning tool, you’ll need to enable your Mac to boot from an external disk, something which is by default disabled on T2 models.

If you do allow a T2 Mac to boot from an external disk, don’t forget to restart in Recovery Mode, open Startup Security Utility, and return its settings to normal, once you’re happy that your migration is complete and your Mac boots properly from its internal storage.

Apple’s rather brief instructions are here, and those for disabling Activation Lock are here.