3 Ways to Fix ‘Couldn’t Unmount Disk’ Error


The user often experiences a problem, while handling some of the Macs, when they are not allowed to repair permissions or directory in Disk Utility. They are unable to erase or partition the drive. Even the Network deployment tools like Apple’s Netinstall service or the DeployStudio fails, on account of this error. The error, which is thrown, is the:

‘Disc erase failed couldn’t unmount disc.’


Disk Erase failed with the error: Couldn’t unmount disk.’

The error occurs in such situations like formatting Mac as well as partitioning and verification and repairing of Mac disk.


The reason for this error is the modification of the boot drive, or it is being used by some other process or application. If the internal drive is causing the issue because the OS X Recovery exists as a partition on the physical disk, unable to modify or unmount the internal drive, then the preference must be given to a network drive or an external drive.


There are many solutions when the user faces these problems.

Using an external drive or OS X Recovery for the starting up of the Mac from another bootable drive. Disk Utility can be run from the external drive. The version of the OS X does not matter, by at least must be 10.7, 10.8 and 10.9. The one and only requirement are that Disk Utility must reside on the drive.

There are three means of resolution of the problem. These are:

(1) Fixing with Partition Manager

If the OS X is denying you to format, erase and repair the Mac hard drive or a volume due to the Couldn’t Unmount Disk’ Error then use Partition Manager Application to format the hard drive with Mac OS X Extended Journaled file system. Formatting the hard drive or partition scheme will recreate new HFS file structure on the hard drive, which will be free from cryptic errors. Further, the disk manager utility will allow the user to create, resize, add partitions on the hard drive with HFS, FAT or NTFS file systems.

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(2) Fixing with USB Boot Drive

This is the most recommended method to repair the error. Any OS X boot drive is needed to complete the task. But, the important point to remember is that the installer or the recovery drive must reside in the drive, separate from the primary boot disk with the installed OS on it.

The following steps can do the fixing:

    • The USB boot drive is attached to the Mac, and the system is rebooted
    • The OPTION key is held during the booting and then the attached boot drive is selected
    • Disk Utility is accessed through the Boot menu. It is the Utilities Menu for the Installer Disk
    • Verification of the Disk is done with the “First Aid.”
    • The repair is done
    • The original task which was halted due to the above error was performed again

The above steps offer a resolution of the problem. The errors could have been unresolvable without a boot drive. Therefore, a bootable thumb drive was a good option. For the older Mac OS X like 10.6 or earlier, the opportunity lies in a SuperDrive and a bootable DVD, and this can also offer the same resolution.

(3) Fixing With the Recovery Partition

The above error may also be triggered due to first Aid or while formatting a non-boot partition. In this case, a resolution of the problem occurs with the Recovery Partition, which comes with all the newer versions of the Mac OS X. However, this is not applicable for the modification of the boot disk with the help of partitions or formatting. Then the resolution lies in the previous method. Otherwise, we follow the steps:

  • The Mac is rebooted, with the OPTION key being held down and the Recovery Partition is chosen
  • The Disk Utility is chosen from the Boot Menu
  • Verification and repairing the disk is done with the “First Aid.”


  • Erase” is used for the disk formatting

This will not work with the disk that is throwing the error is the same as the primary boot partition. The solution lies only in using a separate USB drive for fixing the error.

Resolving Unmounting External USB Drive on Mac

Your USB external hard drive failed to get mount on your Mac? Now, it won’t be possible for you to access the files which resided in the storage medium. Problems such as these keep occurring in a digital environment. If you too have found your hard drive in unmounted condition on Mac and urgently need to recover the data then watch the below video. The video show data recovery on an unmounted external drive with a Mac data recovery utility.
Stellar Data Recovery Professional for Mac supports the recovery of files from an external hard drive on macOS High Sierra and other macOS and OS X variants. Further, it provides support for data recovery on APFS, HFS, HFS+, NTFS, FAT and ex-FAT external hard drives. Once you have registered the Mac recovery tool you can recover as many numbers of files from any given external drive or SSD.
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