Understanding disk utility configuration and data persistence after a factory reset on Mac mini

USArmyVeteran003 wrote:

How could the disk image parent offer the ejection option in the corner? If macOS base system is the recovery hidden OS why could it be ejected and how come there how come the container 5 has physical store name disk4s1 but device disk 5 then the base system under that is disk5s1 and the apple disk image media itself says device disk4 with child count 1. If you can compare what I show in the internal structure with the detailed questions above in relation to that section it would answer my questions. Please bear with me and apologies to the many photos I just really want to get to the bottom of this and learn it. Thank you

https://discussions.apple.com/content/attachment/fd54cb17-155e-416f-930b-a9ecbfd46a33

The disk image resides in ROM, I believe. The user is never expected to interact with it. It seems that Apple have finally done away with revealing it in Disk Utility in the latest macOS version.

As for the naming conventions, don't try to figure that out. It's all logically determined by the OS and relates somehow to the order that these devices become visible to the OS at startup.

I see nothing amiss in the screenshots that you've provided. As example, here is a partial from my own setup on a MacBook Air under Sequoia:

The modern macOS uses a drive format - APFS - vs the older pre-High Sierra OS which used HFS+ (Mac OS Extended) as the format. HFS+ and earlier formats used disk partitions to divide the space on the drive into chunks that could be named and referred to as volumes. 

APFS sets up a drive device (media) with an APFS container (partition) that manages the drive space.  Within the container there will be the APFS volumes that you will see and interact with in Finder and on the desktop for file storage.  Role of Apple File System - Apple Support

The Disk Utility application will display two particular volumes that are required and will be found on any modern macOS installation:

• ”Macintosh HD” is a protected, read-only APFS volume for macOS system files.  There’s nothing you can do with it short of erasing the drive and all the data on it.

• ”Macintosh HD - Data” is the volume where the user accounts and your user data live. Files and folders on this drive may be freely managed by the user accounts that own them.  Together, these volumes comprise a volume group.

Finder always displays this volume group as a single drive on the desktop named “Macintosh HD”, though the volume that you are actually usually interacting with is in this group is Macintosh HD - Data. Finder never reveals the full name of the -Data volume to you, but Disk Utility will.

You will use Disk Utility to manage the APFS formatting of drives and the creation and deletion of volumes within containers.

Disk Utility User Guide for Mac - Apple Support

Add, delete, or erase APFS volumes in Disk Utility on Mac - Apple Support

When you erase and format a drive device with the APFS format, a container is created that will encompass the entire storage capacity of the device. If the device is 1TB, the container is 1TB.

An APFS volume is also created within the container. This named volume is what will be displayed on the desktop when you connect the drive to your Mac.  

Additional containers may be created by partitioning the drive, a non-destructive operation with APFS.

Additional volumes may be added to a container and/or deleted without having to reformat or repartition the drive. These volumes share the whole of the storage capacity of the container and can grow or shrink dynamically as their storage requirements change. 

Adding and deleting APFS volumes is faster, simpler and safer than editing a partition map in older formats.

NOTE: Disk Utility may also display a disk image named “Apple disk image Media” along with the volume “Mac OS Base System”.  This disk image can be safely ignored. User interaction with this is never required and in fact, in the latest versions of macOS 15 that disk image is no longer displayed in Disk Utility.

Like @D.I. Johnson mentioned, it appears you did erase the system since there is only about 6GB of data on the "Data" volume.

There are three ways of erasing the data on the M-series Macs.

• Erase Contents & Settings
• Delete the "Volume Group"
• DFU Firmware Restore (requires access to another Mac currently running macOS 15.x Sequoia)

For the first two options, the following Apple article shows the proper instructions:

Use Disk Utility to erase a Mac with Apple silicon - Apple Support

Here is an Apple article for performing a DFU Firmware Restore which resets the security enclave chip, system firmware, and internal SSD (destroys all data on the internal SSD) and finally pushes a clean copy of macOS onto the internal SSD.

How to revive or restore Mac firmware - Apple Support

This is a factory clean setup. Anything you do during Setup/Migration is what is causing data to fill the drive. If you sign in with an AppleID/iCloud, then macOS enables all iCloud options so it will immediately start downloading stuff from iCloud. Personally I believe this is a terrible default option without asking the user what iCloud options to enable on the Mac.

FYI, There are actually three hidden APFS Containers on an M-series Mac's internal SSD with only one that macOS makes visible to the user ("Macintosh HD" and "Data" volumes). The physical nternal SSD is typically assigned "disk0", and each of those hidden APFS Containers are assigned device identifiers as well "disk1", "disk2", and "disk3" with the latter for the APFS Container holding the "Macintosh HD" & "Data" volumes.

When booting into Recovery Mode, then any volumes needed by the macOS installer are assigned device identifiers as well ("disk4", and "disk5") and perhaps more. Connected external drives can sometimes affect the numbering of these items as well. Just ignore any of the "Disk Images" while booted into Recovery Mode since they are just virtual volumes associated with Recovery Mode and the macOS installer.

You cannot erase the startup drive - that is the drive that you have booted the Mac from.

If you wish to erase the internal startup drive you must reboot the Mac from recovery or from an external drive that has macOS installed.

In the case of reinstalling macOS from recovery, then you must boot the Mac in recovery and use the tool provided on the macOS Utilities page that is presented.

How to reinstall macOS - Apple Support

Another option would be to create a bootable USB drive or install macOS on an external hdd or ssd. These options would have to be prepared before you erase the internal drive, if possible, though I believe macOS may be installed on an external drive while the Mac is booted in recovery..

USArmyVeteran003 wrote:

I have tried cleanmymac, [...]

Uninstall CleanMyMac per the publisher's guidance. It is not needed and is often implicated in Macs that are misbehaving. The macOS can take care of itself without the installation of junkware. CMM provides no useful service to the Mac.

https://macpaw.com/support/cleanmymac/knowledgebase/uninstall-cleanmymac

For more info, please see these support documents:

• Effective Defenses Against Malware - Apple Community

• Avoid phishing emails, fake 'virus' alerts…and other scams - Apple Support

D.I. Johnson wrote:

If you feel compelled to actually manually erase everything from the internal drive - probably unnecessary - you must boot in recovery and use the Disk Utility app in the on the macOS Utilities screen to completely erase and reformat the internal drive. You'd have to select the internal device - APPLE SSD AP0256Z Media - and erase that.

Unless Apple has implemented a safety feature here, people should not erase the whole physical internal SSD of the M-series Macs because it contains two hidden APFS Containers that are necessary for booting to the "Startup Options" screen. I know at one time back in 2020 people were apparently able to erase the whole internal SSD, but I'm not sure that is still possible. I haven't had time to test it with any Sequoia systems.

If those hidden APFS Containers are removed on an M-series Mac, then the only way to fix the M-series Macs is by performing a DFU Firmware Restore using another Mac currently running macOS 15.x Sequoia.

AFAIK, you did provide a link for reinstalling macOS which should have included instructions for a clean install which involves deleting the "Volume Group" which is clearing out the hidden APFS Container holding the actual macOS system files and "Data" volumes.

Thank you, HWTech. I appreciate the knowledge you regularly share here in the ASC. 🙏🏽

I don't disagree with anything in your post. In fact, I would rarely advocate for completely erasing the internal drive, though there are times when it really seems necessary. The recovery tools that Apple have provided are usually plenty to get a Mac back up to speed, I think you'll agree. I'm not fully up to speed with the ins and outs of the M series Mac, but I am trying to keep up! 🙂

I did, in fact, earlier post the link to Apple's guidance for reinstalling macOS from recovery, and in a couple of places it does still reference erasing the startup drive.

❝If the installer doesn't see your disk, your disk might have an issue that requires you to use Disk Utility to erase it. If Disk Utility can’t see it or erase it, your Mac might need service.❞

If erasing the startup drive is detrimental to the volumes required to remain on that drive for the new Macs, I hope Apple will revise their guidance and perhaps make it less likely to dump a Mac into needing the DFU process.

In this thread, I believe the OP has already put the Mac in a position to simply use the Reinstall macOS app on the Utilities window, whether they know it or not. I think the fact that Erase and Reset All Settings left the default volume group on the internal drive was confusing them, especially if they expected the drive to be "completely wiped".

I suppose we'll find out.

USArmyVeteran003 wrote:

[...]

My disk images I want to say are corrupt they aren’t in sequence it’s like one is hidden from the tree. I say this because “Apple SSD” disk0 (seen in photos) then working down it will be like Macintosh disk3s3 and the next list disk not in order etc. do you have a service that helps where I could contact

Your disk is fine. Don't overthink the drive designations.

Trust that DU and the OS are doing their jobs.

In my screenshot below, the disks and volumes are labeled in this order from top to bottom:

APPLE...Media - disk0

Container disk3 - disk3

Mac...SSD volumes - disk3s3s1

Mac...SSD - disk3s3

Mac..SSD snapsh... - disk3s3s1

Mac..SSD - Data - disk3s1

USArmyVeteran003 wrote:

I have a base Mac OS present under disk images 2gb that contains the needed os to reinstall Mac OS after a hard clean via internet recovery is what I’ve read. Not sure why I couldn’t erase SSD. Are you taking a look at the photos I supplied?

You can completely ignore Mac OS Base System and that disk image. There is absolutely no interaction we users ever get to have with that in Disk Utility. Apple really shouldn't even let us see it, and I no longer do see it in my current version of macOS.

Yes, I'm looking at your photos. They all appear normal with the APFS container, the macOS volume group that includes five volumes (Macintosh HD, Macintosh HD - Data, VM, Preboot and Recovery) all normal for a fresh, clean macOS installation.

You have a 256 GB SSD with ~17 GB used and 245 GB free. It appears that you have in fact wiped the drive and now have only macOS 15.4.1 installed on it.

This appears to be a good factory reset. This is the state I would expect to see when the Mac is unboxed for the first time.

What happens now when you restart the Mac mini normally?

I would expect it to step you through the Setup Assistant if you have not already run it.