Didn’t see this one coming but macOS26.4 seems to be dropping HFS+ support.
I found out when opening a DMG that uses this format. You get this error when opening such DMG:
If you use APFS it works fine.
Could be a big issues I guess for a lot of dmg out there.
Hmmm.. known issue. 
HFS external media might fail to mount automatically. (168672160)
Workaround: For macOS only, use CLI tool diskutil mount to attach the relevant disk device.
In my experience diskutil mount fails with DMGs, so you have to use hdiutil attach instead
How can you tell what format a DMG is? Or is this to do with the format of the boot drive that is trying to access a DMG?
Disk utility probably shows it.
Or something in terminal like
hdiutil imageinfo /path/to/file.dmg
