Unable to mount images unless using sudo hdiutil mount from command line

Question

Unable to mount images unless using sudo hdiutil mount from command line

I had some old sparse bundles that I was unable to mount. Double-clicking in Finder, prompt for password, and a message pop up with a yellow exclamation and saying

"The disk image couldn't be opened - Failed to mount filesystems".

Thinking it was corrupt, I tried to go thru repair steps. Turns out ALL my old sparse bundles had the same problem. I then tried to mount a 3rd party DMG file and it mounted correctly to install software. I've been mount 3rd party DMG's fine, it's just the images I create / or created that have issues.

I then tried to create a new non-password sparsebundle, asif, and regular dmg from diskutil, and all three show the same problem. Just empty APFS formatted images, with nothing in them.

After more troubleshooting, it looks like from the command line, I am NOT able to run:

$ hdiutil mount /path/to/Untitled.dmg
hdiutil: mount failed - no mountable file systems

But when I try this:

$ sudo hdiutil mount /path/to/Untitled.dmg
Password:
/dev/disk4          	GUID_partition_scheme          	
/dev/disk4s1        	Apple_APFS                     	
/dev/disk5          	EF57347C-0000-11AA-AA11-0030654	
/dev/disk5s1        	41504653-0000-11AA-AA11-0030654	/Volumes/Untitled

Everything works as expected.

So obviously permissions are messed up somewhere if root can do it but the default login (who is an administrator) is unable. I don't believe it's Tahoe related, but yes - on Tahoe 26.0.1

I have hdiutil mount -debug output for the sudo (worked) and regular user; but not sure how to attach here.

From the debug log towards the end:

2025-10-04 21:25:06.300 diskimages-helper[5294:294306] _postflightMountPointsAfterDAMount: disk5 aborting because no mount point found.
2025-10-04 21:25:06.300 diskimages-helper[5294:294306] _postflightMountPointsAfterDAMount: returned 113

Any thoughts or trouble shooting tips?

Mac Studio, macOS 26.0

Posted on Oct 4, 2025 7:31 PM

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Answer 1

Top-ranking reply

danielfromsomerset Author

Level 1

33 points

Oct 5, 2025 12:45 PM in response to danielfromsomerset

The problem is resolved and I'm detailing for future reference. Hope this helps someone.

Problem Summary: unable to mount an image file. Error messages are few and far between. It just would not mount UNLESS I used the command line: sudo hdiutil attach. I noticed images for 3rd party software had no issues (usually DMG files). It was only image files I had created - sparsebundles, dmg, asif.

Solution: the issue was only with image files hosted on an external volume. I could copy the same image from my external volume to my Downloads folder (where all my 3rd party software images resided) and it mounted as usual. Therefore a permission on the external volume needed to be fixed.

Location and permission information:

/Volumes/ExternalUSBDrive/x/Untitled.dmg had permissions 644 (owned by useraccount/staff)
/Volumes/ExternalUSBDrive had permissions of 755 (owned by root/wheel)

When I made the following change, everything worked as expected:

$ sudo chmod 775 /Volumes/ExternalUSBDrive

It's not obvious to me why the mount permissions for the external volume influence the ability to mount an image residing on that volume. The folder /Volumes/ExternalUSBDrive/x (containing the image) is 755 (useraccount/staff) and the image itself is 644 (useraccount/staff). So I'd assume "useraccount" would have no issue mounting the image. Maybe a wizard can explain.

Thanks for reading.

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Answer 2

Barney-15E

Level 10

122,732 points

Oct 5, 2025 5:55 AM in response to danielfromsomerset

Just a guess, try adding DiskImageMounter to Full Disk Access in Privacy & Security System Settings. DIM is in /System/Library/CoreServices.

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Answer 3

Oct 5, 2025 12:21 PM in response to danielfromsomerset

The plot thickens. It seems the problem is related to the location of the image file. If it resides on my external USB drive - I'm unable to mount unless I sudo. If I copy the image file to my internal drive (Macintosh HD) (I put it in my Downloads folder) - it mounts cleanly without sudo. So it is related to the drive permissions. I'm just not sure what permission to change. I'll poke around.

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Answer 4

Oct 5, 2025 6:22 AM in response to Barney-15E

Thanks for the good suggestion, but that didn't do it. I rebooted after the change as well, and no change. I captured some messages in the Console - and I think it has to do with diskarbitrationd. I see:

unable to mount /dev/disk5s1 (status code 0x00000042).

I captured the exceptions in the following screenshot from Console:

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