Sometimes when formatting a disk, an error occurs that the disk cannot be unmounted. If you use Disk Utility from the system – everything is clear, close all applications using this disk and unmount it manually. But what to do when a similar error occurs when you use the disk utility when restoring the system? In this article I will show how to force unmount USB Boot Drive while “Couldn’t Unmount Disk” error.


Various symptoms that can occur due to Disk Utility could not unmount disk:

  • You can see the end of the system after a while when you start working on it;
  • Your Mac will become too slow or not responsive;
  • You will find mac files and data corrupted without warning;
  • Programs installed on your Mac system suddenly hang or crash during use;
  • You often get a message like “impossible to find a .dmg file”;
  • The problem with the file is not found;
  • You may also receive the message “access denied”.
How to Resolve a “Couldn’t Unmount Disk” Error in Disk Utility?

How to Forcibly Unmount a Disk by Command Line in macOS

Device files in macOS are sets of drivers for specific devices that are stored in the hidden folder “dev”. They are used to interact with these devices through sets of relevant commands (open, read, write, close, and others).

Read more: How to Check Hard Drive Health in macOS?

This allows programs to access devices without the need for the user to initiate each action manually and allows you to use more convenient tools (for example, file managers or Disk Utility). The boot disk on your device will always be numbered first, so the corresponding file will be named “disk0”, and the full path to it will look like: “/dev/disk0”.

You can always see the list of drives connected to your computer by entering a command in the Terminal:

diskutil list

And having learned the name of the file of the disk you are interested in, you can request information about it using the command:

diskutil info /dev/disk1

All that’s left to do is unmount the volume manually:

diskutil unmountDisk force /Volumes/DRIVENAME


As a result, if the problem really was in formatting errors, the problem disk should appear in the Disk Utility as empty, and you can format it and partition it, if necessary, to prepare it for further use macOS.

It’s the best time to ask questions and give us your feedback in comments.


Hi there! My Name is Vincent Lago! My goal is to share insightful reviews, guides, and manuals for people looking to know more about current solutions for Apple hardware and OS. I have years of technological background knowledge including owning a custom iMacs & PCs which was using for different types of business. And now, I want to share my experience with you.

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