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

Jun 14, 2013 - 15 Comments

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.

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

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 OS X it’s for (assuming 10.7, 10.8, and 10.9 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.

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 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 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 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.

Fixing Disk Utility Errors via 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 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 instead.

  • Reboot the Mac holding down the “Option” key and choose the Recovery partition
  • Select “Disk Utility” from the boot menu
  • 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.

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

15 Comments

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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. cookie says:

    Thank you for the auspicious writeup. It if truth be told
    was once a enjoyment account it. Look complicqted to far added
    agreeable from you! However, how could we be iin contact?

  9. 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.

  10. Andrés says:

    Thank you very much!!!!!

  11. uzz says:

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

    DON’T DO ANY OF THE FOLLOWING IF YOU ARE NOT FAMILIAR WITH THE COMMAND LINE AND UNIX – ELSE YOU CAN DAMAGE YOUR COMPUTER SERIOUSLY.

    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

    Then
    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.

  12. 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.

  13. Adrie says:

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

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