MacOS – The document “Example.dmg” could not be opened. Disk Utility cannot open files in the “Disk Image” format

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I've been having trouble with my Mac since I installed Yosemite, where I open an .dmg file, and it would go to the Disk Utility, then I would have to open it from there. My MacBook running Yosemite did not do this, and would give the normal graphic to drag the file over applications to execute it.

Now I've upgraded to Sierra, and I get an alert that says "The document “Example.dmg” could not be opened. Disk Utility cannot open files in the “Disk Image” format". I try to right click the file, but it does not give me an "open with" only an "open" which gives the same error. Again, my MacBook just opens the installer program just fine. I verify the images on Disk Utility, and I get an "operation successful", but the file still won't open.

I am sure there is a way to get round the problem and open the .dmg files, but I would like to adjust the settings so that the installer opens as designed.

Best Answer

So, this solves the problem of how to get the file to open. I don't know why the "open with" was not working, but after several hours of trying different search strings in Google, I found this article that explains how to change the default to the disk image mounter instead of the Disk Utility: How to change default apps on the Mac.

Steps are copied below:

Go to the Mac's Finder.
Find the file type you'd like to open with a specific app, and select it.
Click on the File menu and select Get Info.
Expand Open with: by clicking on the triangle to the left.
Select the Open with: menu, then choose the app you'd like to use to open all documents like that one.
Click the Change All... button.
You'll get a dialogue box asking you to confirm your choice. Click Continue.
Close the Get Info window.

Worked like a champ. I don't know how this got changed in the first place (my Macbook did not change), but that is another question.

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First off, is the disk image a read/write image? Are you able to add new files to it? If not, the instructions below won't work if it is read-only.

The permissions you've listed, "-rw-r--r--@", mean that the file is read and writable by the owner and readable by everyone else. The @ at the end means there are extended attributes to the file, which might include some kind of invisibility option. You would have to get a directory listing with -@ to show the attribute keys. For example, here is a listing showing an image that I manually set to be hidden.

$ ls -l@
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If all your files also have a com.apple.FinderInfo attribute, use Terminal to change into the directory of the files and run the following command to unhide the files.

chflags nohidden *

Now, if your dir listing doesn't show the com.apple.FinderInfo attribute then they are not being hidden using this method but something else is going on.

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MacOS – Disk utility detecting error in single user mode, but not when disk utility is opened via recovery mode

Here's a beginner's guide to the command line. The single user mode is also like looking in as root - the first user that owns all of the computer historically from a unix perspective.

the $ indicates a prompt where you, the human user can type commands. The system is telling you how to prepare the drive for running the repair command.

Type the following text - the same as the first line you are told to from the screen:

/usr/sbin/fsck -fy

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To get out of that screen, you could halt the Mac or type exit to resume the normal start up process.

Apple documents the root user and fsck in these two articles if you are curious or wish to learn more:

https://support.apple.com/en-us/HT204012 - About the root user
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https://support.apple.com/kb/PH21893 - Single user mode in El Capitan

Best Answer

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