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According to Apple,

"macOS Monterey includes Erase All Content and Settings, a way to quickly and securely erase all of your settings, data, and apps, while maintaining the operating system currently installed."

Does anyone know the details about what happens in the background and if it differs from erasing the drive via Disk Utility in Recovery Mode and reinstalling macOS?

Extra info: Apple put my M1 MacBook Pro in an out-of-box state recently when they were servicing it. I used Migration Assistant to restore from a Time Machine backup, and since then have been experiencing a number of bugs with various apps (both native & 3rd party) and iCloud syncing. So I'd like to just rebuild this machine from scratch. I'm wondering if Erase All Content and Settings might leave any trace system settings or files that were created from or since restoring via Migration Assistant.

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By default, macOS creates two separate volumes on your disk: "Macintosh HD" and Macintosh HD - Data". (Since Catalina.)

The first contains the Operating System files, including apps like Mail, TextEdit, etc. It is normally read-only, and cryptographically sealed, so that any modification will be flagged.

The second volume contains all your data: not only user accounts, but system prefs and settings, files added to /Library, third-party apps, caches, etc, etc.

Normally, macOS presents the two volumes 'as one', hiding the division in the Finder.

Consequently, erasing the "Data" volume will remove any user-defined settings, parameters, software, and anything else. This is what Erase All Contents does.
There is no need or point in erasing the OS volume, because its contents have not changed.

Erasing All Contents and Settings is a broad tool for fixing a problem. I would only do that after trying other trouble-shooting techniques, such as testing in a new user account; testing in Safe Boot mode; signing out and in of AppleID, etc, etc.

You could even try just restoring the entire Data volume from your TM backup, rather than 'migrating' from it.

Modern disk formats are increasingly complex: there are Volume Groups, Containers, Logical Volumes and more. The internal disk also contains the recovery partition, EFI firmware partition, etc.

The concept of "I'll just wipe the disk and start again" is a bit 90s. ;-)

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  • Great, this answers my question, thanks! M1 machines don't let you restore a volume with Time Machine from Recovery Mode. And for what it's worth, I've indeed tried everything you mentioned, and more, such as removing every trace of a couple problem apps (using AppCleaner, which finds every system and /Library file that was installed with the app) and then re-installing them, only to find the same problems and crashes. I've had much success over the past ten years with an occasional wipe-and-rebuild approach (which is in fact still commonly suggested by Apple in a situation like this).
    – gills
    Commented Jun 29, 2022 at 16:36
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    AppCleaner is over-rated: it can leave stuff and delete the wrong stuff too! Are you sure the crashes are not bugs in the software, perhaps caused by old apps running on Apple Silicon?
    – benwiggy
    Commented Jun 29, 2022 at 16:41
  • It's certainly more thorough than just deleting the .app file. Do you have a better suggestion to "re-install" an app? If AppCleaner isn't thorough, wiping and rebuilding certainly would be. As far as bugs go, many apps that utilize iCloud, both native and 3rd party, are exhibiting bugs (non-functioning buttons, blank screens, crashing if you click on the wrong thing or resize a window, etc). All of these apps are Apple Silicon supported. eg. Bear, Spark, Photos, Reminders, Calendar, etc.
    – gills
    Commented Jun 29, 2022 at 16:55
  • Can you indicate clearly since which version has macOS done what you describe, creating two volumes and then merging them together for display purposes? That is relatively new behavior (perhaps since Catalina?), but your answer pretends/reads as if it's been this way forever.
    – Cody Gray
    Commented Jun 29, 2022 at 20:28
  • @CodyGray-onstrike macOS 10.15 Catalina was the first version of macOS to separate the drive into a read-only System volume and a writeable Data volume, using the ability of the APFS file system to support two volumes in a single Container (functionally, a partition). However, the current model, in which the read-only components are stored in a cryptographically sealed APFS snapshot (known as the Signed System Volume), rather than an ordinary APFS volume, began with Catalina's successor, macOS 11 Big Sur. Finder plays tricks to show it as one, but you can see the split in Terminal.
    – Ivan X
    Commented Oct 8, 2023 at 17:49

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