Resolve a “Couldn’t Unmount Disk” Error in Disk Utility

Jun 14, 2013 - 25 Comments

Couldn't Unmount Disk Error as seen in Disk Utility on a Mac

Disk Utility usually works trouble-free, but a frustrating “Couldn’t Unmount Disk” error can halt whatever the attempted task is right in its tracks. This can happen during partitioning, disk verification and repair, and even during formatting, and there’s usually little to no additional information provided as to how to resolve the problem or even what the problem is with regards to the error message or the app in Mac OS.

Typically the “Couldn’t Unmount Disk” error pops up when the currently boot drive is being modified, thus the easiest solution is to boot from another drive and run Disk Utility from there instead. For the boot drive, it shouldn’t matter which version of Mac OS X it’s for (assuming 10.7, 10.8, 10.9, 10.10, 10.12, 10.13, etc at least), the only requirement is that it has Disk Utility – which they all do. This will allow you to fix the problem, regardless of the cause, by one of two means, the first is a sure-thing to fix the issue, while the other only works sometimes. We’ll cover both with a bit of explanation. We’ll also show you a way to forcibly unmount a disk by command line, though that approach must be used with caution as it can lead to data loss on the drive in question.

How to Resolve the Unmount Error with a USB Boot Drive

This is the recommended method because it should always fix the error. You will need any Mac OS X boot drive to complete this task, I used a Mavericks boot installer drive for this purpose but others should work too, whether they are installation drives or just recovery drives, the important thing is they are bootable and separate from the primary boot disk that stores the installed OS:

  • Attach the USB boot drive to the Mac and reboot
  • Hold down the OPTION key during boot, then select the attached boot drive (typically has an orange icon at the boot menu)
  • At the boot menu, choose “Disk Utility” (if using an Installer disk, pull down the “Utilities” menu to access Disk Utility)
  • Go to “First Aid” and verify the disk, then repair if needed
  • Now perform the original task that threw the “Couldn’t Unmount” error

I ran into this twice recently, first when attempting to modify partitions on a drive, which came right along with a separate “partition failed” error, and again was triggered when attempting to format those partitions. The above steps did the trick and everything was working again as expected.

This is a good example of why it’s very valuable to have a bootable USB thumb drive set up with whatever version of Mac OS X is running on your Macs, because without a separate boot drive some of these errors would be unresolvable. Such boot drives are easy to create on your own, here are instructions for making boot disks for OS X 10.9, OS X 10.8, and OS X 10.7. For older Macs running prior versions of Mac OS X, typically anything running OS X 10.6 or earlier will have a SuperDrive, and thus shipped with a bootable DVD that can serve this same purpose.

How to Fix Disk Utility Errors via Mac Recovery Partition

If the Unable to Unmount Error is triggered by first aid or formatting a non-boot partition, you may be able to fix the error by booting from the Recovery partition that is included with all new versions of Mac OS X. This will not work if the error was triggered by attempting to modify the boot disk through partitions or formatting, and you will need to use the method above with a boot disk instead.

  1. Reboot the Mac holding down the “Option” key and choose the Recovery partition
  2. Select “Disk Utility” from the boot menu
  3. Go to “First Aid” to verify and repair the disk, or go to “Erase” to format the disk

Again, if the disk throwing the errors is the same as the primary boot partition that Recovery is also on, the above method may not work to resolve the problem. In that case, you’ll need to boot from a separate USB drive to fix the error.

How to Forcibly Unmount a Disk by Command Line in Mac OS

Another method uses the command line to force unmount a disk, but this is not the top recommended option because of potential for data loss.

Caution must be used with this approach however because forcibly unmounting a disk can cause data loss of the drive being forcibly unmounted. Thus this is only appropriate if you plan on formatting and erasing the disk to you are force ejecting anyway.

From the command line of Mac OS, enter the following string:

diskutil unmountDisk force /Volumes/DRIVENAME

Replace “DRIVENAME” with the name of the volume you want to unmount, then hit RETURN key to force the drive to unmount.

Do you know of another solution that can resolve the “Couldn’t unmount disk” error message in Disk Utility? Share your experiences and solutions in the comments below!

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Posted by: Paul Horowitz in Mac OS X, Tips & Tricks, Troubleshooting


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  1. Sheriff says:

    This really helped me. The fist step has worked perfectly. Thank you so much.

  2. Jak says:

    Much appreciation! This was driving me crazy.

  3. abdul rahaman says:

    I want to install couldn’t

  4. Gary Bridger says:

    Ok I guess I done this right, I have taken lion osx from dvd dragged it to usb stick. So now yor saying open mac by pressing said key. Open disc utility find my external drive , that will not un mount or partition. And hopefully fix it from there?

  5. Monish d kothari says:

    Thanks a tonne for the help… It resolved my issues ….

  6. Monish d kothari says:

    For people unable to boot from other disk ( elaborated method) :
    * Download the OS X firmware form App Store
    * take a hdd and format it to Mac OS X extended journal
    * create a guid table partition
    * then install the OS X on it
    * now pressing opt n reboot the device
    * now select the external hd recovery mode
    * now you will be able to format your mac Hd in disk utility

  7. Shunshine says:

    Thanks so much…needed it quickly and your site was informative.

  8. TE says:

    Couldn’t get around this problem until I found your solution… Used diskmaker x to make an el capital boot disk then followed your steps and it worked on my 2011 MacBook Pro with OS X Lion. Many thanks.

  9. Andrés says:

    Thank you very much!!!!!

  10. uzz says:

    You don’t need any boot drive nor recovery partition utility to fix this error.


    Open the command line and type the following:

    diskutil list
    (press enter to execute the command)

    You will see something like:

    0: GUID_partition_scheme *8.0 GB disk1

    Find your disk by size and name – in this case it’s disk1

    Then type
    diskutil unmountDisk /dev/disk1

    sudo dd if=/dev/random of=/dev/disk1 bs=4096

    REPLACE disk1 WITH YOUR disk here that you will see in the distil list output.

    The last command will prompt for password.

    Press Ctrl+C to abort execution of the last command in a minute (it’s enough to just fill the beginning of the disk with random garbage).

    Open the disk untility and eras your disk normally – the error will be gone.

  11. John says:

    You can force unmount a drive by running the following Terminal command:

    diskutil unmountDisk force /Volumes/VOLUMENAME
    Replace VOLUMENAME with the name of a volume on the disk you are trying to unmount.

    Following this, attempt to Erase/Partition the drive again in Disk Utility. Using the above command can interrupt file read/writes, which can cause file corruption. However, since you are erasing the drive anyway, this doesn’t really matter in your situation.

  12. Adrie says:

    Great answer. Booted from a boot usb and it did the trick. I love you so much

  13. chris says:

    totally saved my bacon, thanks guys!

  14. Alex says:

    Thank you very much!!

  15. Vince says:

    After wrangling with this for a while I happened across a very easy fix (this is based on OS 10.9 Mavericks, your version may differ slightly):

    1. In Disk Utility choose your drive on the left (the first listing for the drive itself, not the one below it with the drive’s assigned name)
    2. Choose the “Erase” tab in the main window
    3. Click the “Security Options” button
    4. Select the second most secure option just to the right of “Fastest” (in earlier versions of DU I think it included options like “write zeros once”, “write zeros 35 times”, etc.), then click OK
    5. Click the “Erase” button
    6. THE KEY STEP: When DU starts erasing the drive and writing zeros, a “Skip” button appears. Click it (unless you really do want to write zeros, which can take a LONG time). Your drive will now be unmountable!

    Hope this helps someone else, it sure saved my bacon!

  16. Matt says:

    Nothing you suggested worked. In the end I gave up and booted off an Ubuntu USB, deleted the partition and reformatted as EXT4. Rebooted with the High Sierra USB install and off it went. Why does Apple make things so bloody difficult?


    • surah says:


      do you have any specific tips on doing this? I’m in a situation where nothing is working also. I am able to run linux mint off a usb, but I am new to it so I don’t want to make a wrong move.

      thanks so much,

  17. Patrick Harris says:

    Try renaming the usb drive and then using disk utilities.

    Worked for me and since it’s so simple you may be willing to try it. I do hope it works for you.

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