• Disk images
  • Troubleshooting

no mountable file systems: why a disk image will not open, and how to tell a bad download from a full disk

A DMG that will not open usually downloaded incompletely, often because the disk ran out of space. How to check it in seconds, and what else causes it.

4 min read

You double click a .dmg and macOS says The following disk images couldn't be opened with the reason no mountable file systems. Most of the time the file is incomplete or damaged, and the most common reason for that is a download that ran out of space or was interrupted.

Check the file in ten seconds

ls -lh ~/Downloads/*.dmg
hdiutil verify ~/Downloads/YourFile.dmg

Compare the size with the size the website states. If hdiutil verify reports a checksum failure, the file is damaged. Delete it and download it again.

If the vendor publishes a SHA-256 checksum, that is the definitive test:

shasum -a 256 ~/Downloads/YourFile.dmg

If the download keeps failing

  • Check free space first. A browser writing a 12 GB installer to a Mac with 8 GB free gives up quietly and leaves a broken file. Our guide to what is safe to delete gets you room quickly.
  • Download with a different browser, or with curl -LO from Terminal, which resumes with -C -.
  • Turn off any VPN or content filter that may be cutting the connection.

If the file is fine and still will not mount

  • The image uses a newer format. An APFS disk image will not open on very old versions of macOS.
  • Too many images are attached. Check with hdiutil info and eject the ones you forgot.
  • Try attaching it from Terminal, which prints the real error:
hdiutil attach -verbose ~/Downloads/YourFile.dmg

Once it is installed, the DMG itself is just clutter. Our guide to disk images taking up space shows how to find the old ones.

Common questions

What does no mountable file systems mean on a Mac?

macOS could not find a readable file system inside the disk image. Usually the download is incomplete or damaged, often because the disk filled up or the connection dropped.

How do I check if a DMG file is damaged?

Run hdiutil verify followed by the file's path in Terminal. If the vendor publishes a SHA-256 checksum, compare it with the output of shasum -a 256.

Can a full disk cause a DMG not to open?

Yes, indirectly. A download that runs out of space can stop partway and leave a file that looks complete but is not, which then fails to mount.

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