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.
- 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.
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!
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