“Couldn’t unmount disk” errors usually appear in Disk Utility when the boot drive is busy. As a first line step be sure to close all open files and programs and try again. If the error persists, reboot the computer and try again.
Using a boot drive
If you’re still having problems there are a couple of options to fix this error but the easiest is usually to boot from another drive and run Disk Utility from there. This completely stops the boot drive from running (since you are using a new boot drive) and so the issue can’t present.
Once you’ve got your boot drive, run Disk Utility and use the ‘First Aid’ option to verify the disk and repair if needed. Now attempt the task that caused the error again. Full step by step instructions for this process are available here.
In some cases you can use the Mac recovery partition instead.
How to use the Mac recovery partition
- Reboot into recovery mode by holding Command and R whilst the Mac is starting up
- Click disk utility from the boot menu
- Click first aid to verify and repair or erase to format
If the error presented through modifying the boot partition, use the boot drive method instead as this one won’t work.
Unmount via command line
You can try to unmount the drive from the command line. You can access the terminal from recovery mode, it’s under Utilities -> Terminal.
Caution: This method can cause data loss
Open the terminal application and enter the following command replacing [DRIVE_NAME] with the name of the drive you are trying to unmount.
diskutil unmountDisk force /Volumes/[DRIVE_NAME]
If that command doesn’t work try using:
Find the drive in the list and make a note of the identifier. It will look like ‘disk’ with a number after it. E.g. disk3. When you have your identifier use the following command replacing [disk_id] with your identifier:
sudo diskutil unmountDisk force /dev/[disk_id]