Monterey boot disk structure

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Level 7

25,388 points

Monterey boot disk structure

I would like to understand the nature of the boot disk under Monterey.

After reading Boot Disk Layout in macOS Monterey and doing additional research I thought I had gained a reasonable understanding of it. But then I looked at my own iMac running Monterey ... it's a plain vanilla 2019 i9 Intel iMac (no T2 chip).

When booted normally into macOS Monterey, in Disk Utility, I see the following: APPLE SSD SM1024G Media (the system drive), Container disk 1 (the APFS container), volumes Macintosh HD & Macintosh HD - Data ... and the Macintosh HD snapshot (the sealed, bootable base system) inside Macintosh HD. All looks as I expected it to look.

However, when I boot into Recovery Mode and run Disk Utility from there, I see this instead:

Here, the macOS Base System is shown in what appears to be a Disk Image, and not inside Macintosh HD. And it appears to be in a separate Container disk. Is this an additional Container (aka partition) on APPLE SSE SM1024G Media?

Why are these two views different? ... and where does the macOS base system ("snapshot") actually reside?

iMac

Posted on Feb 10, 2023 1:20 PM

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Answer 1

Barney-15E

Level 10

123,038 points

Feb 11, 2023 6:47 AM in response to MartinR

macOS Base System is Recovery, not part of the normal boot process, and it is a mounted image, not an actual structure.

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Answer 2

MartinR Author

Level 7

25,388 points

Feb 11, 2023 6:11 AM in response to Barney-15E

I don't see anything wrong, per se. I'd like to know why the macOS Base System is shown in two different locations. I would have expected both instances of Disk Utility to show it in the same location.

In the first screenshot (booted in Monterey) the macOS Base System is shown as a snapshot existing within the Macintosh HD volume.
In the second screenshot (booted in Recovery), the macOS Base System appears to be in its own container in separate media.

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Answer 3

MartinR Author

Level 7

25,388 points

Feb 11, 2023 6:36 AM in response to MartinR

Pls. ignore my previous reply. Apparently I was editing for too long a time. Here's what I meant to say:

I don't see anything wrong, per se.

What I'd like to understand is why the macOS Base System is shown in two different locations. I would have expected both instances of Disk Utility to show it in the same location.

In the first screenshot (booted in Monterey) the SSD has one Container (Container disk1) and the macOS Base System is shown as a snapshot existing within the Macintosh HD volume inside Container disk1.
In the second screenshot (booted in Recovery), the SSD still has one Container (now called Container disk3) and the macOS Base System is not shown inside it. Why is the snapshot not there? Instead, there is an additional Media - "Apple disk image Media," as "Container disk2" with the macOS Base System inside it.

Is the disk image "Apple disk image Media" inside Container disk1 (aka Container disk3) or is it in a separate container (aka partition) on the SSD?

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Answer 4

MartinR Author

Level 7

25,388 points

Feb 13, 2023 11:18 AM in response to Barney-15E

Got it! Thanks ! Very helpful to know this. FWIW, I also checked this in Recovery on my Catalina iMac and it shows the macOS Base system disk image mounted. (Never took notice of that before, that's why it stood out when I looked more closely under Monterey.)

Still curious as to why the Macintosh HD snapshot does not show up in Disk Utility in Recovery Mode, however.

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