dmg not recognized

I'm not real sure what you are asking, so this is a guess on my part.

Select any .dmg file. Press COMMAND-I to open the Get Info window. In the Open With panel use the dropdown menu to select DiskImageMounter then click on the Change All button. If DiskImageMounter is not displayed in the dropdown menu then select Other. Navigate in the Open dialog to the /System/Library/CoreServices/ folder where you will find the application. Select then click on the Open button in the dialog.

Select a disk image file in the Finder and open the Info window. From the Open with: popup menu, select DiskImageMounter, then click Change All...

That typically means either a bad download or you are trying to open an image file for Windows.

Does this happen with files that you had already downloaded and were able to open before the update? If not, do the files have zero size? Also, please post the exact text of the message you see, or a screenshot.

Have you installed "Speed Download" or anything of that sort?

The files are corrupt. I can't say why. Usually it's a failed download.

Press "apple/command J" to open your downlowds window and ensure that the dmg file is completely and successfully finished. Otherwise, if the downloading is not finished and you try to open directly from your downloads directory it will prompt an error message

For me it says: No mountable file systems, Whaaaat? 😕

Hi.

Open DiskUtility / Images / Convert

choose broken file (unmountable dmg image) and convert it to "DVD/CD master"

After ends you can mount it like a charm.

thats end