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

Jun 14, 2013 - 40 Comments

Couldnt 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, or if a disk was trying to be erased you may find the erase failed with a couldn’t unmount disk error. For the former situation where the boot drive is being modified, 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, 10.14, 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.

If that doesn’t work, you can take this a step further:

You might also need to target the disk by device identifier to forcibly unmount it, in which case you can first find the disk with:

diskutil list

Then when you find the matching disk to the identifier (/dev/disk1, /dev/disk2, /dev/disk3, etc), you can target the disk to unmount as so. For the example syntax here we’ll use /dev/disk3 to forcibly unmount from command line, and using sudo which will gain superuser privileges for the task:

sudo diskutil unmountDisk force /dev/disk3

Hit return and enter the admin password to forcibly unmount the disk from the Mac.

When finished you can quit out of Terminal as usual.

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!


Related articles:

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


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  1. Kevin W. Parker says:

    None of these worked for me, including the “sudo … force”. I entered my password, and it’s just sitting there.

    • Paul says:

      You have to hit the return/enter key after typing your password in the command line, the password won’t show up for security reasons just hit enter when done.

      Hope that helps!

  2. ES says:

    Thanks for your help!

  3. Benny says:

    This was the most well thought out and articulated explanation ever. Thank you.

  4. Anonymous says:

    Hey, nothing is working in these suggestions. I am trying to wipe my desktop Mac, so I chose the last option. Evidently, there is a GUID_partition_scheme that is 1 TB big listed under /dev/disk0 with the identifier disk0 that “could not be unmounted”. Does anyone know how to fix this? If so, that would be greatly appreciated.

  5. Sima4 says:

    Sudo commnd not found error please help

    • Alisson says:

      Did you managed to to unmount the drive, because I’ve the same problem and still can’t do anything
      sudo command not found

  6. Pulpy says:

    I realised I had a file open that meant it couldn’t unmount. Just in case anyone finds this…

  7. P chandu says:

    Hi MacBook a1181 Mac os X 10.7.5 lion factory reset disk1s3 read write message disk locked how this factory reset possible this model

  8. Franco Partoo says:

    thank you so mucho, the one that work for me was using the terminal

  9. Sidney Goldstein says:

    Restarting the computer solves the problem. Then disk utility will repair or do whatever I want. Problem is that this is a recurring issue in mojave on my fusion drive. Wish there was a permanent solution.

  10. Scotty d says:

    I got this error on an external FireWire drive that is used primarily for backing up my 2011 iMac. We had a power outage on 1/12/2019 and this apparently caused some issues with the external drive. Backups didn’t happen, and I looked and found the drive had not mounted. I powered the drive off and powered it on again and it mounted. I attempted to use the drive but it was damaged. So I pulled up diskutil (the GUI version) and tried running it to give the drive “First Aid” and that’s when I got this error. I used the command line:

    unmountDisk force /dev/disk3

    To force the unmount of the disk, and now First Aid is proceeding. Will have to see whether it’s successful. It’s been checking the catalog file for a while now.

  11. 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.

  12. 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,

  13. 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!

    • Todd says:

      Awesome Hack. Worked like a charm and quickly. I had been trying all sorts fro things and this was the best one. Thanks

    • ar1 says:

      Great workaround. here is what I did for macOS Catalina

      Selected the second option from VIEW.
      1. Then it shows the the folder containing my drive
      2. Selected the parent folder for “Macintosh HD” and and all the options worked. I was able to erase my Drive and continue with restore.

  14. Alex says:

    Thank you very much!!

  15. chris says:

    totally saved my bacon, thanks guys!

  16. Adrie says:

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

  17. 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.

  18. 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.

  19. Andrés says:

    Thank you very much!!!!!

  20. 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.

  21. Shunshine says:

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

  22. 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

  23. Monish d kothari says:

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

  24. 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?

  25. abdul rahaman says:

    I want to install couldn’t

  26. Jak says:

    Much appreciation! This was driving me crazy.

  27. Sheriff says:

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

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