First Aid on Physical Disk Fails ?!!

Question

Level 1

12 points

First Aid on Physical Disk Fails ?!!

I'm somewhat at my wits end having spent the last month with some sort of First Aid issue on my work 27" iMac Intel 2020....

It was saying that the data volume First Aid would just fail. So I thought the data was corrupt and bought a new Studio to replace it thinking I could then Erase everything on this iMac and install fresh OS in Recovery and repurpose this for my personal use.

All seemed okay, migrated my old personal Time Machine over to this newly Erased iMac, and things seemed to be fine.... Until my 4th Super Duper backup failed.

So I ran Recovery First Aid, and this time while the Data, Volumes and Container are all fine - now it says First Aid on the very top Physical Disk "Apple SSD AP2048N Media" failed - repeatedly.

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

Does this mean basically that the Hard drive in the iMac is toast? It would be a shame as I love this big Nano screen 27" iMac and its only 5 years old... sob

Any ideas?

Thanks!

iMac 27″, macOS 15.5

Posted on Jun 7, 2025 4:16 PM

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Answer 1

Jun 8, 2025 7:34 AM in response to lilskye2

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

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Answer 2

Jun 8, 2025 8:18 AM in response to Matti Haveri

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

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Answer 3

John Galt

Level 10

151,866 points

Jun 7, 2025 4:39 PM in response to lilskye2

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.

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Answer 4

John Galt

Level 10

151,866 points

Jun 7, 2025 6:47 PM in response to lilskye2

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.

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Answer 5

Jun 8, 2025 12:04 PM in response to lilskye2

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.

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Answer 6

lilskye2 Author

Level 1

12 points

Jun 8, 2025 9:57 AM in response to Matti Haveri

Thanks everyone for your replies -

To answer John Galt - The reason I ran DU is because this "office" Mac was being erased and replaced because it was having DU failures at the Data level and a pvm in its trash that wouldn't erase (doubling the size of everything).

So after replacing it with a new Mac Studio, I wanted to convert this 27" Intel T2 iMac to use as my personal.

So after following instructions for "Erase all Contents & Settings" and installing new Sequoia and my personal info from my TM BU, I wanted to run DU and make sure it was now all okay.

It was - except for this weird "Couldn't mount disk (-69842)" on the overall SSD (Mac HD).

It didn't seem to matter for 2-3 days until my 3rd day of scheduled SuperDuper BU failed.

I use SuperDuper to make up daily BUs to an external drive in addition to using Time Machine hourly (to a different external drive.)

Been doing this for years on all my Macs.

On this new personal Mac, all went well for the first 2 days of SD BUs. Until yesterday.

Suddenly the scheduled SD BU failed (repeatedly) stating : "No such file or directory"

So I booted into Recovery and Ran DU First Aid on everything to see what's up. And the result is what I originally posted - that the main HD "Problems were found with the partition map which might prevent booting. Couldn't mount disk: (-69842)"

So I'd like to get to the bottom of this and make this Mac healthy and working properly so I don't encounter mania down the road.

Just recapping that this Mac was my office Mac and had DU failures at the Data level, which I assumed would be resolved by using "Erase All Contents and Settings" and starting as a new Mac with completely different Data. I'm learning now that apparently didn't really Erase everything.

So I just want to be sure there isn't something inherently wrong with this Mac HD itself now just manifesting in another way before I continue entering my personal life and have it go sideways down the road.

UPDATE - After seeing all the replies this morning (thank you all!), I am seeing I am not alone! Other people are also seeing this same DU error on their overall SSD Mac HD ! I didn't know if it was that the drive was failing or if its a combo of having to Erase and Install Sequoia (when this had been running Ventura).

I appreciate Matti Haveri linking to the discussion of Fernando N. here: Disk Utility error "Couldn't mount disk (… - Apple Community This sounds exactly like what's happening with me. (And I also think this was caused by an original "ghost" partition that was left over from the undeletable pvm in the trash that somehow this still sees as a partition its looking for - but what do I know?)

I will admit his solution with the word "destroy" in a Terminal command sounds terrifying.

But I'm willing to do whatever to resolve this to keep this 27" nano screen iMac :)

I have never used Internet Recovery - just so I am clear:

I am on an Intel with a T2 chip.

If I go to Terminal and put:

gpt destroy /dev/disk0

diskutil eraseDisk APFS "Macintosh HD" /dev/disk0

It will erase the entire Mac HD. Am I doing this from just "Terminal" or am I doing this from Internet Recovery?

Once you completely "destroy" this - I am not clear how you are actually functioning on the Mac to be in Internet Recovery to complete the rest of this. Or are the actual basics still there, that will let you be in Internet Recovery? LOL!

Sorry to be such a dolt, but appreciate all this help and hopefully it will help others!

Thank you all.

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Answer 7

Jack-19

Level 8

46,134 points

Jun 7, 2025 4:54 PM in response to lilskye2

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

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Answer 8

Jun 8, 2025 10:43 AM in response to lilskye2

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

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Answer 9

lilskye2 Author

Level 1

12 points

Jun 7, 2025 5:52 PM in response to John Galt

Thanks for your quick reply.

Yes I have already done all these steps a few times and at least my data is backed up on Time Machine (and I still have the original personal 2018 iMac that I was replacing with this one. So all is not lost.

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?

It is sounding more to me like this disk is dying or corrupted/damaged in some way. And if I understand correctly there's no way for me to use the iMac by instead booting from an external drive as it is a T2 (though I see people claim I can change some security setting in Recovery to allow this???)

I'm willing to try anything to save the 27" iMac and wish they still made these!!!!!!!!!

I will await your comments to this before I attempt to again reinstall the OS.

Thanks!