macOS Error Opening .dmg Disk Image - No Mountable File Systems

I ran into an interesting macOS error while working with a customer a couple of weeks ago. I didn't find a lot of good search results addressing the issue, so I decided to write up a post about it myself.

The error was as the screenshot above shows; trying to open a dmg (disk image), macOS showed the error "no mountable file systems". If you see the "no mountable file systems error" while opening a dmg, here's what you should try:

1. In most cases, the downloaded dmg file is actually corrupt or had an error downloading. If possible, try downloading the dmg again, turning off any download assistant plug-ins you may have. You can try downloading the file in a different browser as well. Or if you don't need to be logged in to the site to download the file and you want to be fancy, you can try curl -O url in Terminal to download the file. (There's an example of that in my screenshot below.)

2. Reboot your Mac if you haven't already tried that. Apparently there is an issue sometimes after opening too many dmg files, that is fixed with a reboot.

3. Try mounting the DMG on the command line in Terminal. We will at least get some sort of useful error message to go on if it still fails:

   • Open Terminal: In Spotlight, the search magnifying glass at the upper right corner of your screen, search for Terminal, and press enter to open the Terminal app.

   • Type hdiutil attach into the terminal. Don't press enter yet.

   • Drag the dmg file from your Finder window onto the Terminal window and let go. This will fill in the location of the dmg file into your Terminal window.

   • Press enter.

4. Think about if you have any kind of security policies on this machine to prevent writing to external drives (thumb drives, optical drives, etc). I haven't seen this one in action, but I read about this being a possibility while researching the issue.