macOS Sierra claims all DMG files I download are damaged

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macOS Sierra claims all DMG files I download are damaged

Any time I try to mount a DMG file (by double-clicking on it in Finder) I get the following error message:

"<filename>.dmg is damaged and can’t be opened. You should eject the disk."

If I use File / Open Disk Image in Disk Utility or the third-party app DropDMG to mount the .DMG file then it works just fine, and I can access the contents of the disk image.

So, why is there a problem in Finder? Has anyone else had this problem? It's very irritating.

Macbook, Mac OS X (10.5.2), N/A

Posted on Dec 5, 2016 3:57 AM

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Answer 1

Best reply

John Galt

Level 10

140,878 points

Dec 5, 2016 3:29 PM in response to Meglos

That's correct. There are a couple of plausible explanations. Either the downloads are becoming corrupted by something (ill-conceived "cleaning" and / or "anti-virus" junk comes to mind), or macOS has been corrupted, likely by the same causes. Fix them by uninstalling such things, and reinstalling macOS, respectively.

It's also possible if your Mac's date and time are not correct, leading to signed applications not installing correctly. Fix that using System Preferences > Date & Time.

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Answer 2

John Galt

Level 10

140,878 points

Dec 5, 2016 10:06 AM in response to Meglos

Control-click the .dmg file and select Open With >

What is shown adjacent to "Default"?

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Answer 3

Meglos Author

Level 1

9 points

Dec 5, 2016 12:09 PM in response to John Galt

Thanks for responding.

The default app shown for opening DMGs is DiskImageMounter.app

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Answer 4

dialabrain

Level 10

122,465 points

Dec 5, 2016 12:14 PM in response to Meglos

Just curious, do you have any anti-virus programs or other junk like CleanMyMac installed? They are known to cause various issues.

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Answer 5

Meglos Author

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9 points

Dec 5, 2016 1:51 PM in response to dialabrain

No, I don't have anti-virus or CleanMyMac installed.

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Answer 6

Meglos Author

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Dec 5, 2016 2:00 PM in response to John Galt

Thanks for the suggestion.

I have reinstalled macOS, and that hasn't helped unfortunately (I downloaded a new DMG to make sure, that it wasn't just existing ones - before I did the reinstall - that were affected)

My Mac's date and time are correct, and being synced automagically with Apple's time server.

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Answer 7

Meglos Author

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9 points

Dec 5, 2016 1:59 PM in response to John Galt

In case it's relevant, I copied some of the DMGs I have been having trouble with from my Mac onto my MacBook.

There they have no problem at all being opened. So I don't think the DMGs are corrupted.

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Answer 8

John Galt

Level 10

140,878 points

Dec 5, 2016 3:28 PM in response to Meglos

Please shut down your Mac and restart it in "Safe Mode" according to the instructions in the following Apple Support document: Try safe mode if your Mac doesn't finish starting up - Apple Support.

Determine if the .dmg files open normally in "Safe Mode".

Then, restart your Mac normally.

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Answer 9

Meglos Author

Level 1

9 points

Dec 5, 2016 3:28 PM in response to John Galt

Thanks for the suggestion John.

I think I have found where the conflict is. I run a piece of software called BoxCryptor on my download folder where the DMG files are held. WHen I move the DMG files out of that folder they run properly.

I discovered this after booting in Safe mode (which, can I say, isn't the easiest thing to do with a wireless keyboard - would be great if Apple gave you an earlier visual clue that Safe mode had been entered)

I run BoxCryptor on my MacBook too, but the problem doesn't seem to be occuring there. Anyway, I'm comfortable that BoxCryptor is causing the issue so I'll be in touch with its developer.

Thanks again.

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