Apple Silicon firmware and disk structure

[AppleTO]

So, I just picked up the base M1 MacBook Air. I am glad I did... I love it!


I am trying to warp my head around the new firmware and disk structure on these (that I guess replaces the UEFI).

Previously, if I wanted to reformat and reinstall a fresh copy of macOS, I would boot Internet Recovery and then reformat the primary drive as a new GUID with APFS.

Now, Internet Recovery no longer exists in the same format and you must use the built in macOS recovery. From what I understand (based on some conflicting articles on the internet), if you erase and reformat the entire disk the Mac with become completely unresponsive and you will need to revive/restore it with another Mac (because the firmware for the Mac itself lives in the first container of the APFS filesystem - which is hidden and only about 500MB).

The second container contains the 'Macintosh HD' volume(s) and is safe to erase (the whole container can be erased, not just the Macintosh HD(s)). If you do this and reboot, it brings you right to the 'Activate Mac' dialogue and then after to Recovery. I just tried this and reinstalled macOS with no issues. As a side note, it is incredibly fast to install macOS from a previously erased volume. This 'Recovery' must be the in the third container.

The third and last container (also hidden) is called Apple_APFS_Recovery and it seems this is the recovery image that launches when you hold down the power button while booting and select 'Options' (or automatically if you wipe your Macintosh HD container). As this container is only 5.4GB, is seems the full macOS image is not actually in it. You need to connect to the internet so the latest version can be downloaded. In a way, this is sort of like Internet Recovery.

The activate Mac part is interesting. An erased 'Macintosh HD' volume (with an OS previously on it), seems to trigger this.

To my understanding, you can still create a bootable macOS USB installer and boot from it to install macOS, but you still cannot erase the full disk because you will wipe the firmware off the machine. I have not tried this, however.

Anyway, for anyone who knows more about this, please let me know if I have made any mistakes here. I am coming from the 2015 13' MacBook Pro.

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***Important Note***

It seems like the above steps to erase and reinstall macOS may end up trigging what seems to be a bug in user account creation. When you try to create a user account, macOS may freeze for a while and then refuse to make the account.

Instead of directly erasing your ‘Macintosh HD’ volume or container, launch terminal and type “resetpassword” and press enter. This will launch an app and the terminal will continue to run in the background. Click the ‘Recovery Assistant’ menu at the top of the screen and then “Erase Mac”. Click ‘Erase Mac’ on the app and then continue. The machine will then restart and need to be activated.

This Erase Mac function appears to erase the ‘Macintosh HD’ volume, but it must also clear some things in the firmware container.

After you activate the machine you may want to erase/rename the volume or container again manually because the Recovery Assistant names the volume ‘Untitled’. Not very nice.

I am not sure if this is a bug or not, but this whole process seems kind of convoluted. I hope they improve on this.

 

[jdb8167]

Unless you like to play around with recovery, I wouldn't recommend erasing your Macintosh HD volume. There are security settings that live in the volume and when they are removed you have to jump through hoops to recover. It is flakey and has many issues.

If you do decide to mess around with this stuff because you need or just want to understand it, the first thing you should do is create a bootable installer following the directions here: How to create a bootable installer. This will give you a way to boot your Air if everything goes wrong, which it certainly might. The fact that I had a bootable installer on a thumb drive was the one thing that saved me when I was messing around with this stuff.

Edit: I did completely erase the internal SSD and recover from the bootable installer. I couldn't recover from the mess that recovery made when I tried to use resetpassword without wiping the disk and starting over.

 

[Quackers]

The first time I wiped the disc with resetpassword and erase mac (repeated) the re-installation went fine.
The second time I did it (to wipe the disc to return the MBP to Apple) I ended up resetting the password and erasing the mac 4 times because it would not load the normal recovery option screen. It kept booting into a loop asking for me to choose language then activate mac and then rebooting.
On the 4th time it then loaded the recovery options and re-install ran normally. I nearly had to send it back empty.
So the process can be buggy.
But at least I know how to do it now (and what can go wrong). It might come in handy for my incoming MBA
So will the 11.1 USB installer I made.