First Aid on Physical Disk Fails ?!!

lilskye2 wrote:

"Problems were found with the partition map which might prevent booting. Couldn't mount disk: (-69842)

I get the same alert when booted to Recovery or to a bootable macOS 15.5 install USB. But somehow NOT when booted to Internet Recovery and running its Disk First Aid.

For now I have ignored that alert and just kept doing regular Carbon Copy Cloner backups as usual. I might reformat the internal drive and try to fix it when doing the next upgrade (although Sequoia might be the last supported macOS for Mac mini 2018).

A while back someone in these forums recommended temporarily formatting a device as MSDOS-FAT (MBR), and then as APFS (GUID) so a potentially corrupted partition table is fixed. I have done that when doing a clean install from a bootable macOS installer to an Intel Mac mini 2018. See also another possible option to fix that:

Disk Utility error "Couldn't mount disk (… - Apple Community

Matti Haveri wrote:

…

A while back someone in these forums recommended temporarily formatting a device as MSDOS-FAT (MBR), and then as APFS (GUID) so a potentially corrupted partition table is fixed. I have done that when doing a clean install from a bootable macOS installer to an Intel Mac mini 2018…

That would be surprising, but a corrupt GPT can undoubtedly manifest in various ways.

That workaround would have to be targeting a corrupt GPT, too.

GPT includes a so-called protective MBR, and GPT is a layout built upon and extended from the MBR design. In GPT, the four partitions of the protective MBR contains the entirety of the device, marked as GPT storage using the type 0xEE. (Use of the type 0xEF was around for a while in some related contexts, too.)

Apple has a good technical description of the “disk partitioning scheme known as the GUID partition table, or GPT” but alas searches for “GPT” are now deeply buried in AI-related discussions.

Here’s the structure: Technical Note TN2166: Secrets of the GPT

Refer to Repair a storage device in Disk Utility on Mac - Apple Support.

• Ignore Step 1 because you will be performing that "repair" on the boot device. Begin with the Note in Step 2.

Then, follow each one of the remaining steps in the provided order.

If you already did all that, then proceed below the horizontal rule.

That's all you can do.

I will say that this will be my third Erase all Contents and reinstall of Mac OS (not sure if this actually "reformats" the disk or if that is something I have to purposely do somehow? I presume that is what DU does? Yes?

To be clear, if you arrived at this step in Repair a storage device in Disk Utility on Mac:

• If Disk Utility can’t repair your disk, or you receive a report that the First Aid process failed, try to repair the disk or partition again. If that doesn’t work, back up as much of your data as possible, reformat the disk, ...

... what that's telling you to do is to Erase and reformat a storage device in Disk Utility on Mac.

Specifically, follow the instructions described in "If you want to erase your startup disk."

But that may be getting ahead of ourselves. Start at the very beginning. Was there a reason you ran Disk Utility to begin with?

In other words, what was it that led to your original concern?

The reason for asking is that if DU reported "Problems were found with the partition map which might prevent booting" yet the Mac apparently boots and runs ok, then I recommend you do absolutely nothing — other than maintaining Time Machine backups of course. Please clarify that before taking any more action. I'll explain later.

lilskye2 wrote:

Is this different from "Internet Recovery" - do you mean just from Command +R, or Option+Command+R

Or is there no difference?

Command-R (oldest available macOS) and Option-Command-R (newest available macOS) are how Recovery is started, sometimes called Internet Recovery.

Brief history: Recovery was once booted from local storage (and that worked well, so long as local storage didn’t get corrupted or erased), then the ability to also boot macOS over the ‘net became available with what Apple termed Internet Recovery (and that was useful should local recovery be unavailable or erased), then it was all lumped back under the name Recovery. The older Macs are decidedly less good at Recovery and that for various reasons, but most Macs that have once run macOS starting around 10.12 do fairly well, here.

And just to add a little bit more info for the sake of muddy clarification, Apple appears to also sometimes use the term macOS Recovery to denote booting from the Mac's local recovery system with Command-R vs Internet Recovery (Command-Option-R and Shift-Command-Option-R) as you have described.

How to start up from macOS Recovery - Apple Support

Hey there!

I'd suggest running DriveDx to check the health of the SSD. The trial edition is enough to run a check.

Let us know if any errors are shown.

-Jack

You have to use DU booted from Recovery (or from a different bootable device) to erase the Mac boot storage.

If their recent changes to that Support document were intended to clarify the various options, they accomplished the opposite.

Any confusion will become moot as macOS inexorably becomes indistinguishable from iOS. No DU First Aid, no Time Machine. No options. Simple!

If the internal drive turns up flaky.

You can run that iMac from an external USB-C or Thunderbolt SSD.

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