Notarized .dmg won't open

To be clear, it’s failing Gatekeeper when you try to mount the disk image?

If so, I don’t think that Gatekeeper has even looked at your app yet, so there’s something wrong with the disk image itself. What do you see with the following:

% codesign -v -vvv /path/to/your.dmg

Oh, and one other thing, try re-testing on a frest machine, per Testing a Notarised Product. Sometimes Gatekeeper can tie itself up in knots on your main development machine.

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