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06 Numbers and Powers

Disk Images Couldn’t Be Opened: Fixes

Card No. 06·67287 · Updated · Standard formula, shown below

If you’re seeing “Following disk images couldn’t be opened” on your Mac, it means macOS’s DiskImageMounter failed to mount one or more .dmg files. This usually happens due to a corrupted download, an incompatible image format, or a security setting, and the fix is often as simple as re-downloading the file or adjusting your Gatekeeper preferences.

What Causes This Error?

The error message appears when you double-click a .dmg file and macOS cannot mount it as a virtual disk. Common causes include:

  • Incomplete download: The file didn’t finish downloading or was interrupted, leaving it truncated.
  • Corrupted file: Data corruption during download or transfer (e.g., via USB or cloud sync).
  • Incompatible format: The .dmg was created for a different architecture (e.g., Intel vs. Apple Silicon) or uses an unsupported compression method.
  • Security settings: Gatekeeper may block the image if it’s from an unidentified developer.
  • Outdated macOS: Older versions of macOS may not support newer .dmg formats.
  • Damaged file system: Rarely, the disk itself has errors.

How to Fix It: Step-by-Step

  1. Re-download the file: Delete the .dmg and download it again from the official source. Ensure the download completes fully.
  2. Check the file size: Compare the file size on your Mac to the size listed on the website. A mismatch indicates an incomplete download.
  3. Try a different browser: Sometimes browsers cache corrupt downloads. Use Safari, Chrome, or Firefox as an alternative.
  4. Right-click and select Open: Instead of double-clicking, right-click (or Control-click) the .dmg and choose Open. This sometimes bypasses Gatekeeper.
  5. Adjust Gatekeeper settings: Go to System Settings → Privacy & Security. Scroll down and look for a message about the blocked app. Click Open Anyway if available.
  6. Use Disk Utility to verify: Open Disk Utility, go to File → Open Disk Image, and select the .dmg. This may give a more specific error.
  7. Check for macOS updates: Go to System Settings → General → Software Update and install any available updates.

What Is a Disk Image?

A disk image (.dmg) is a single file that contains the complete structure of a disk or volume. It’s like a virtual hard drive that can be mounted and accessed as if it were a physical disk. macOS uses .dmg files for distributing software, because they preserve file permissions and can be compressed. When you double-click a .dmg, macOS mounts it, and you can drag the application into your Applications folder. After installation, you can eject the virtual disk.

Common Misconceptions

“The file is infected with a virus.” While malware can be distributed via disk images, the error itself is not a sign of infection. It’s usually a technical issue.

“I need to delete the file.” Not necessarily. The error often means the file is unusable, but you can try the fixes above before giving up.

“It’s a problem with my Mac.” Usually it’s the file, not your system. But if all .dmg files fail, then it could be a system issue.

Advanced Troubleshooting

If the basic steps fail, try these:

  • Use the command line: Open Terminal and run hdiutil attach /path/to/file.dmg. This will show detailed error messages.
  • Check the file type: Use file /path/to/file.dmg to see if it’s actually a disk image or something else (like a zip file).
  • Verify checksum: If the download page provides a SHA-1 or SHA-256 checksum, use Terminal’s shasum command to verify the file integrity.
  • Repair disk permissions: Boot into Recovery Mode (hold Command+R during startup) and use Disk Utility to run First Aid on your startup disk.

Prevention Tips

  • Always download from official sources.
  • Use a download manager for large files.
  • Keep your macOS updated.
  • If you’re transferring files, use a reliable method (e.g., direct cable, not Wi-Fi).

Frequently Asked Questions

Why does the error say “following disk images couldn’t be opened” in plural?

macOS groups multiple failed mounts into one message. If you tried to open several .dmg files at once, they all failed for the same reason. Fix one, and the message may disappear.

Can I open a .dmg on Windows?

No, Windows does not natively support .dmg files. You can use third-party tools like 7-Zip to extract the contents, but you cannot mount it as a disk.

What if the .dmg is for an older Mac?

If the .dmg was created for PowerPC or early Intel Macs, it may not be compatible with modern macOS. Check the software’s system requirements.

Is it safe to use a .dmg from an unknown source?

No, only download from trusted sources. If you must use an unknown .dmg, scan it with an antivirus and check its code signature using codesign -v in Terminal.

How do I eject a mounted disk image?

Drag the mounted volume icon to the Trash, or right-click it and select Eject. You can also use the keyboard shortcut Command+E.

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