So the first thing to do is to startup the Mac from another bootable drive such as an external drive or OS X Recovery. Normally, any ‘Couldn’t Unmount Disk’ error is attributed to circumstances where the boot drive is being modified or is being used by an application or process.
I have found myself recently experiencing a couple of Macs which would not allow me to repair the directory or permissions in Disk Utility, or erase/partition the drive with an error such as: ‘Disc erase failed couldn’t unmount disc’ or ‘Disk Erase failed with the error: Couldn’t unmount disk.’Įven trying to use Network Deployment tools such as Apple’s Netinstall service or DeployStudio have also failed to deploy due to these errors.
