Steps to Resolve Error Preparing the Time Machine Disk and Couldn't Unmount Disk on macOS

Description

There was an error preparing the Time Machine disk. Couldn't unmount disk when a drive erase fails.
This happens when the disk is being used by something.

Resolution

1. Click Apple Icon.
   Click System Settings...
   
   


2. Type Time Machine in the search box.
   Click Time Machine.
   
   


3. Click Options...
   
   


4. Set the Backup Frequency.
   Select Manually.
   Click Done.
   
   


5. Power OFF the computer.
   Remove the drive.
   

   Remove the power adapter on external Desktop Drives. Wait 5 seconds. Plug the power adapter into the drive.



6. Power ON the computer.
   Connect the drive.


7. Open Disk Utility.
   
   Need more help?
   Answer ID 3823: How to Access macOS Disk Utility


8. Erase and format the drive.
   
   Need more help?
   Answer ID 8200: Steps to Format a Drive for Use with Windows or macOS


9. Setup Time Machine backup.
   
   Need more help?
   Answer ID 18952: Time Machine Backup and Restore from macOS to a WD External Drive

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