Cannot access DMG file created by Disk Utility

It doesn't need to be converted. Did you try to convert it?

Was it accessible before you converted it?

Have you tried running First Aid on the drive?

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

Hmm, after doing an initial Internet search, I came across this Wikipedia entry:

With the introduction of Mac OS X 10.6, Apple dropped support for formatting or writing HFS disks and images, which remain supported as read-only volumes.[1] Starting with macOS 10.15, HFS disks can no longer be read.

Ref: https://en.wikipedia.org/wiki/Hierarchical_File_System

If this is true, then the only potential solutions I see are:

1. Use a Mac, running a pre-macOS Catalina OS, or
2. Run a VM of a pre-macOS Catalina on your current Mac to try to access it.

Tesserax wrote:

Hmm, after doing an initial Internet search, I came across this Wikipedia entry:

With the introduction of Mac OS X 10.6, Apple dropped support for formatting or writing HFS disks and images, which remain supported as read-only volumes.[1] Starting with macOS 10.15, HFS disks can no longer be read.

Ref: https://en.wikipedia.org/wiki/Hierarchical_File_System

I'm pretty certain that reference is to HFS not HFS+.

You are correct. Thank you! FWIW, continuing my research, I have come across numerous instances of folks having this same issue, here at the ASC and other sources. Unfortunately, none appear to having a working solution ... so far.

If the drive is failing, you should stop messing with it and take it to a data recovery company.

If it is failing or has failed, nothing you do will likely recover the files.