Disk image won't open after installing macOS Tahoe 26.2

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Disk image won't open after installing macOS Tahoe 26.2

I used Disk Utility to create a password protected disk image several years ago and use it to store sensitive information. Since updating to Tahoe 26.2 when I try to open the disk image I get the following error message:

The disk image couldn't be opened

Failed to mount filesystems

I verified that my username has RW access to the disk image file.

I am able to open a copy of the disk image on a different laptop running Sequoia 15.7.

I was happily accessing the disk image under the previous version of Tahoe.

Not sure what to try next on this one, so any help would be appreciated.

Posted on Jan 12, 2026 3:22 PM

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Answer 1

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neuroanatomist

Level 9

66,925 points

Jan 12, 2026 7:32 PM in response to sjmarbu

The problems I have seen reported have been with sparse disk images, and I suspect that is due to the fact that with Tahoe, Apple replaced the sparse bundle format (UDSB) with a new Apple Sparse Image format (ASIF).

Personally, I've had no issues with encrypted disk images created years ago working in Tahoe, but I use read/write (now called 'RAW' images that allocate a fixed space block for the image. If I find the contents approaching the capacity, I make a new image and copy the files over.

In your case, I'd suggest opening the image on the Sequoia Mac, and copying them to a new image on the Tahoe Mac.

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Answer 2

Jan 12, 2026 5:54 PM in response to sjmarbu

You should move away from using encrypted DMGs. I have seen multiple posts recently where people have discovered their older DMGs can no longer be opened when using Tahoe.

I really hope you have good backups of the data you have stored in these DMG files and not just a backup copy of the DMG itself just in case the backup copy of the DMG has become corrupted/damaged.

These days it is better to create a new encrypted APFS volume to store those items.

You can even configure macOS to ignore that encrypted volume so you are not prompted to unlock it every time the system boots, but you will need to use Disk Utility to manually unlock it when needed. Here is an article with the instructions to prevent automatically mounting the encrypted APFS volume at boot.....the only change needed is in Step#8 where you need to change "hfs" to "apfs" in the line entered into the "fstab" file. I've been using this setup for a little while now.

Prevent a volume from mounting at startup - Apple Community

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