Disk Utility on your Mac allows you to manage internal disks and external storage devices. Usually, Disk Utility works well on your Mac and performs tasks just as you command, but sometimes an error saying “Couldn’t Unmount Disk” can stop the whole process.

This error can stop partitioning, disk verification, repair, and formatting. This can be hard to troubleshoot because you’re not provided with any extra information. When you unmount a disk on Mac, it’s made inaccessible by your PC. This is a software process that stops the macOS from being able to read and write data to the disk.

The main reason why the “Couldn’t Unmount Disk” error shows up in Disk Utility when the boot drive is being modified and is suddenly stopped by another process. This can in your drive and be fixed with a few troubleshooting steps.

1. Boot from USB Drive

To troubleshoot this problem, you’ll need a Mac boot drive. Since the error was caused by the boot drive being modified, you should try and boot from another drive and run Disk Utility from this. The requirements for the boot drive: it needs to have Disk Utility, it needs to be bootable, and it needs to be separate from the primary drive that contains the Mac OS.

1. Enter the USB boot drive to your Mac.

2. Reboot your Mac and hold Option key while it’s booting.

3. Select the attached boot drive.

4. Choose Disk Utility at the menu.

5. Go to First Aid and verify the disk. From here, you can repair the disk if you need to.

6. Perform the task to the disk. This is the task that you first did before the error.

2. Force Unmount Disk

If you want to troubleshoot the error that stopped the unmounting process on your disk, you can force unmount the disk using Terminal. This will force your Mac to give up it’s ability to read and write to your disk.

1. Go to Finder > Utilities > Terminal. Or click Command + Space bar to open Spotlight search and search for Terminal.

2. Enter the following command:

diskutil unmountDisk force /Volumes/VOLUMENAME

3. Click Enter.

You need to replace VOLUMENAME with the name of a volume on the disk that failed to unmount.

4. Go back to Disk Utility and Erase or Partition the drive. You need to repeat the same steps that you took before you receive the error now that you’ve entered the command in Terminal.

The command in the Terminal can interrupt file read/write which can cause the drive to corrupt. But since you’re unmounting the drive, corrupting it won’t matter.

3. Recovery Partition

If you’ve tried troubleshooting the error but nothing has worked, you can try and fix the error by booting from your Macs Recovery partition. If you tried to modify your boot disk by partitioning or formatting, booting from the recovery partition won’t work.

1. Reboot your Mac and hold Option key while it’s booting.

2. Select the Recovery partition.

3. Choose Disk Utility from the menu.

4. Go to First Aid to verify and repair the disk.

5. Perform the task to the disk. This is the task that you first did before the Couldn’t Unmount Disk error.

If the disk that caused the error the same as the primary boot partition that the Recovery partition is also on, using the Recovery partition might not fix the error.

4. Force Restart Mac

If there’s a software issue with your Mac that causing the error to show up when trying to unmount your disk, it can cause the operating to fail. To troubleshoot this, you can force restart your Mac to get rid of any software issue that causing the unmounting task to fail with the drive.

To force restart your Mac, hold Control + Command while pressing the Power button. This combination will force restart your Mac. From here, you can erase or partition your disk in First Aid and the “Couldn’t Unmount Disk” error shouldn’t stop the operating.