How to Fix "Couldn’t Unmount Disk" Error on Mac?

by Jake Doe - -
12

Question

Issue: How to Fix "Couldn’t Unmount Disk" Error on Mac?

I was trying to erase my drive on MacBook, but I received an error message saying “Disk Erase failed with the error: Couldn't unmount disk.” What does it mean?

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Solved Answer

“Couldn’t Unmount Disk” error is an error message that might pop up on your Mac’s screen when the device is not allowed to repair particular permissions or directories in Disk Utility,[1] for instance when trying to repair a hard drive. When a device cannot erase or partition the drive, users are delivered one of these error messages:

Disc erase failed couldn’t unmount disc.

or

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

There are several reasons why users cannot complete necessary tasks because of this error. Usually, the source of the error is particular boot drive[2] which is modified or corrupted. The alert might also pop up when the driver is being used by other apps or processes. Sometimes, the issue is an internal drive.

Fix "Couldn’t Unmount Disk" Error on Mac

Due to the “Couldn’t Unmount Disk” error users might encounter these problems as well:

  • Inability to partition Mac HFS;
  • Inability to open particular files or programs;
  • Lost or corrupted data on the hard drive;
  • Sudden elimination of the running application.

However, the error might be triggered by malware as well. Nevertheless, Mac OS X is pretty resistant to various cyber threats;[3] security experts warn about an increase of Mac malware[4] that might sneak inside the device and might trigger various errors and problems. Therefore, if noticed other symptoms of the virus attack, you should obtain a professional security software, such as MacCleanse, and scan the system with an updated tool.

If virus attack was not the problem, try the methods below that will help to fix “Couldn’t Unmount Disk” error.

How to deal with “Couldn’t Unmount Disk” error

Example of "Couldn’t Unmount Disk" Error

There are two main ways that can help to solve “Couldn’t Unmount Disk” error on Mac computers. The first method seems to work in the majority of cases. However, if it does not give expected results and the error shows up again, the second way should help to deal with the issue.

Method 1. Check USB Boot Drive

  1. Make sure that USB boot drive is attached to the device.
  2. Reboot the system. When rebooting hold the Option key. From the appeared menu select the attached boot drive.
  3. Access Disk Utility from the appeared Boot menu (however, if you use Installer disk, go to Utilities menu and click Disk Utility option).
  4. Go to First aid. Verify the disk and repair it if it’s necessary.

Once it’s done, try to complete the same task again. If the “Couldn’t Unmount Disk” error shows up again, try the second method.

Method 2. Use Recovery Partition

This method should help if the “Couldn’t Unmount Disk” error occurred when formatting a non-boot partition or caused by a first aid. The Recovery partition is included in all new OS X versions. Therefore, follow these steps:

  1. Reboot Mac. When rebooting hold down the Option key.
  2. From the appeared menu choose Recovery partition option.
  3. In the appeared boot menu choose Disk Utility.
    – Go to First Aid if you want to verify and repair the disk.
    – Go to Erase if you want to format the disk.

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References

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