I have downloaded a dmg file but when I click on it it keeps verifying.

I've also tried to go to /System/Library/CoreServices/Installer.app but I can't click on the .dmg because it is greyed out.

How can I open it?


4 Answers 4


The command line tools allow you to mount the DMG and skip verification.

Open Terminal:

hdiutil attach -noverify /Path/To/Your/DMGFile.dmg

If you have the DMG file on your Desktop, and the DMG was named "Broken.DMG" you could type in something like this:

hdiutil attach -noverify ~/Desktop/Broken.dmg

Rather than typing the name of the file, just drop the file icon in the terminal window once the main command is ready. If your image is bad, the warning is correct and you should get a new dmg rather than bypass the error checking step.

A more global change would use defaults, but this check is very handy in general since it flags bad images:

defaults write com.apple.frameworks.diskimages skip-verify TRUE

This will stop the verification of DMG files permanently. After you've managed to skip the verification and open that file, it would be a good idea to re-enable the verification by opening Terminal again and typing in:

defaults write com.apple.frameworks.diskimages skip-verify FALSE
  • IMO, the "Verifying file.dmg" dialog needs a skip button. I have a very large image file (50GB) on a NAS volume that takes several hours to verify (via SMB on a 2020 MBP). The fact that my only options are to wait or globally disable Gatekeeper every time I want to access the image is depressing.
    – DaveL17
    Jul 3, 2021 at 17:18

The verification is used to make sure the .dmg package is not corrupted. You had better wait for the verification to be done, since the likely situation is that your file is bad, not that the check can’t run.

If you really wanna skip the verification, execute the following commands in the Terminal,

# Disable the "Are you sure you want to open this application?" dialog
defaults write com.apple.LaunchServices LSQuarantine -bool false

# Disable Disk Image Verification: Verifying ...
defaults write com.apple.frameworks.diskimages skip-verify -bool true
defaults write com.apple.frameworks.diskimages skip-verify-locked -bool true
defaults write com.apple.frameworks.diskimages skip-verify-remote -bool true

To reverse the settings,

defaults delete com.apple.LaunchServices LSQuarantine -bool false

defaults delete com.apple.frameworks.diskimages skip-verify
defaults delete com.apple.frameworks.diskimages skip-verify-locked
defaults delete com.apple.frameworks.diskimages skip-verify-remote



An alternate to the other answers (which don't work in all cases) is to turn off gatekeeper. This lets you open a dmg without the annoyingly slooooow verify step.

This has the additional benefit (in my opinion) of removing the restriction on what apps you can install. Of course there is a risk if you aren't careful, but hey, its my/ your computer, so up to me/you if you want to take the risk.....

So the terminal command to turn off gatekeeper is

sudo spctl --master-disable

to re-enable if you want

sudo spctl --master-enable

Check status by

spctl --status

Addendum: For some reason, even with gatekeeper turned off, verify still occurs on some (not all!) dmgs that I have loaded down from websites - but there doesn't seem to be a pattern - different dmgs from the same website - one will verify, the other skip the verify and go straight to mounting. All the dmgs that I have created for my data open without verifying. Intriguing......

  • I've also found that some files still require verify--including DMGs I've created for my own data. In my case, it could be because the DMGs are located on a NAS rather than stored locally. Quite frustrating. I have one DMG that takes hours to complete.
    – DaveL17
    Aug 11, 2021 at 18:26


FastDMG is a free app available from https://sveinbjorn.org/fastdmg which I have used for this for a long time.¹ It’s very convenient, and if the .dmg is corrupted, it won’t mount, so that will be the sign that something might be wrong. Also, if FastDMG does fail to mount a .dmg for some reason, it will offer to send it to the default macOS Disk Image Mounter.

¹ Because this is the Internet, allow me to add: “YMMV. Use at your own risk. No warranty expressed or implied.”

Features (copied from the current web page)

  • Doesn’t waste your precious time verifying disk images
  • Auto-accepts annoying end-user license agreements
  • Runs in the background (doesn’t show up in the Dock)
  • Displays no windows or menus and stays out of your way
  • Multithreaded, can concurrently mount several images
  • Disk image document icons continue to look the same
  • Handles all disk image types supported by DiskImageMounter (dmg, iso, toast, etc.)
  • FastDMG is actually a minimal wrapper around the hdiutil command-line tool that ships with macOS and therefore uses Apple’s private DiskImages framework indirectly.

(I’ll also add: you can set it to be the default app to open .dmg files, or you can just select a .dmg file in Finder and do: right-click » Open With » FastDMG for some DMGs if you with.)

The app is free with donations requested if you find the app useful.


Download from https://sveinbjorn.org/files/software/FastDMG.zip

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