Disk Utility User Guide
Launch the Disk Utility app on your Mac and highlight the hard drive icon under ‘ External’ (for external hard drives). If you choose any other drive, the partition option will become unclickable. Now click Partition on the top toolbar. A window that has partition information will pop up. You can force unmount a drive by running the following Terminal command: diskutil unmountDisk force /Volumes/VOLUMENAME Replace VOLUMENAME with the name of a volume on the disk you are trying to unmount. Following this, attempt to Erase/Partition the drive again in Disk Utility.
You can unmount a disk set or a member disk if you need to repair it or perform other maintenance.
Unmount a disk set
In the Disk Utility app on your Mac, select the disk set that you want to unmount in the sidebar.
Click the Unmount button in the toolbar or beside the disk set name.
After unmounting a disk set, you can disconnect the disk members of the set from your computer.
Unmount a disk member of a set
In the Disk Utility app on your Mac, select the disk set that you want to unmount in the sidebar.
Click the Unmount button in the toolbar or beside the disk set name.
Disconnect the disk member that you want to unmount.
Mac Disk Utility Unable To Unmount Volume For Repair Mac
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If you unmount and disconnect a disk and then reconnect the disk later, Disk Utility rebuilds the disk member automatically if you selected “Automatically rebuild” when creating the disk set. If you need to rebuild the disk manually, see Repair a disk in a disk set.