Does the new "Erase all content and settings" feature reinstall macOS? ("Macintosh HD")

[collin_]

So macOS, by default, has two volumes: Macintosh HD and Macintosh HD - Data. Macintosh HD is the operating system whereas Data contains all the user data. Most users don't do anything that would make any changes Macintosh HD, but I have done a couple things that would (through sudo in Terminal).

I cannot find a clear answer on whether the "Erase all content and settings" feature "regenerates" Macintosh HD. There are hints that it would (like Apple's article telling you to do it instead of reinstalling macOS the old way before selling or giving away your Mac), but then I also see articles explicitly claiming that it is not a full reinstall.

Anyone have the answer to this? Thanks.

 

[anshuvorty]

So macOS, by default, has two volumes: Macintosh HD and Macintosh HD - Data. Macintosh HD is the operating system whereas Data contains all the user data. Most users don't do anything that would make any changes Macintosh HD, but I have done a couple things that would (through sudo in Terminal).

I cannot find a clear answer on whether the "Erase all content and settings" feature "regenerates" Macintosh HD. There are hints that it would (like Apple's article telling you to do it instead of reinstalling macOS the old way before selling or giving away your Mac), but then I also see articles explicitly claiming that it is not a full reinstall.

Anyone have the answer to this? Thanks.

Why don't you give it a try in a VM?

 

[BigBlur]

No, it doesn't reinstall macOS. Macintosh HD is a read-only sealed/signed volume and only contains the OS. None of your data is on there, so there is no sense for it to reinstall or regenerate Macintosh HD. It only needs to wipe the 'Macintosh HD - Data' volume, where your files and data live. It's essentially the same as how it's done in iOS.

Most users don't do anything that would make any changes Macintosh HD, but I have done a couple things that would (through sudo in Terminal).

It's likely whatever you did, didn't do anything to 'Macintosh HD' unless you're leaving out info. Just sudo in Terminal alone cannot do it. Did you do the whole rigmarole of disabling SSV authentication, mounting the volume, and bless it after you made the changes (and leaving SSV authentication disabled)? If not, macOS would fail to boot if the volume was modified. This also would have needed to be done after every macOS update, because the updates overwrite or replace the system volume.

 

[maxsquared]

So macOS, by default, has two volumes: Macintosh HD and Macintosh HD - Data. Macintosh HD is the operating system whereas Data contains all the user data. Most users don't do anything that would make any changes Macintosh HD, but I have done a couple things that would (through sudo in Terminal).

I cannot find a clear answer on whether the "Erase all content and settings" feature "regenerates" Macintosh HD. There are hints that it would (like Apple's article telling you to do it instead of reinstalling macOS the old way before selling or giving away your Mac), but then I also see articles explicitly claiming that it is not a full reinstall.

Anyone have the answer to this? Thanks.

Nothing you sudo install would be there, it's like after you reformat and install the OS to the latest version installed without the reformat and install your OS, the whole process takes a couple of minutes.

For reselling, I guess there is quick formatting and proper formatting, what I've experienced when quick formatting, some services still "remembers" things from settings, for example, 1password account detail is retained after quick format and reinstall, where proper format (the one takes ages) would wipe the account details and etc as well, I tend to format like this when I am selling my MacBook. The erase all content and settings behave like quick format + reinstalling.

 

[rm5]

Why don't you give it a try in a VM?

I've tried it in a VM, and it does NOT work! Sticks at a black screen after rebooting to the Apple logo a few times. Probably has something to do with it not bring real hardware and no real chip. Tried it in both Parallels and the Xcode Monterey VM.

Tried it on my M1 MBA last month, and it worked beautifully! Very quick process, took under 5 minutes.

 

[TriBruin]

I've tried it in a VM, and it does NOT work! Sticks at a black screen after rebooting to the Apple logo a few times. Probably has something to do with it not bring real hardware and no real chip. Tried it in both Parallels and the Xcode Monterey VM.

Tried it on my M1 MBA last month, and it worked beautifully! Very quick process, took under 5 minutes.

It won't work in a VM as EACAS requires either Apple Silicon or a T2 chip. VMs do not support or emulate the T2 chip.

 

[collin_]

No, it doesn't reinstall macOS. Macintosh HD is a read-only sealed/signed volume and only contains the OS. None of your data is on there, so there is no sense for it to reinstall or regenerate Macintosh HD. It only needs to wipe the 'Macintosh HD - Data' volume, where your files and data live. It's essentially the same as how it's done in iOS.


It's likely whatever you did, didn't do anything to 'Macintosh HD' unless you're leaving out info. Just sudo in Terminal alone cannot do it. Did you do the whole rigmarole of disabling SSV authentication, mounting the volume, and bless it after you made the changes (and leaving SSV authentication disabled)? If not, macOS would fail to boot if the volume was modified. This also would have needed to be done after every macOS update, because the updates overwrite or replace the system volume.

Ohhhhhhh you're right. I had a fundamental misunderstanding here. Thank you. Was also able to verify by trying to make a txt file somewhere in the system volume and it was like nope.

Semi-related question: Does Apple's newer way of doing things (with the separate read-only system volume) make clean installs unnecessary?

I occasionally reinstall macOS, partially because it's very easy in my case (password manager helps a lot). I do it partially to clean up junk I no longer need (which just 'Erase all content and settings' would handle), but also because I have this theory that after a system volume has been updated enough times, it might get kind of... messy. Idk, operating systems are just so huge and I can't even imagine how much care Apple has to take to make sure nothing gets fudged up when macOS updates -- especially with major under-the-hood changes (e.g. the jump to APFS a while back).

Could my theory that macOS can get "messed up" a little be true, or is Apple (on T2 and M1 devices) running some sort of check verifying that every single line of code compromising macOS is exactly what it should be, and/or just deleting the system volume entirely and replacing it with the latest version when it updates?

 

[chabig]

Does Apple's newer way of doing things (with the separate read-only system volume) make clean installs unnecessary?

Yes it absolutely does. The system volume cannot be changed, modified, or corrupted.

 

[chabig]

…is Apple (on T2 and M1 devices) running some sort of check verifying that every single line of code compromising macOS is exactly what it should be…

Yes. This is exactly what the signed sealed system volume does.