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.’ Even trying to use Network Deployment tools such as Apple’s Netinstall service or Deploy Studio have also failed to deploy due to these errors.