Also perform a permissions repair if required. Select the ‘First Aid’ tab and verify the troublesome disk, repairing if needed.

If you have created your own bootable drive with a full system, open Disk Utility from /Applications/Utilities.

If you have used our method above to create a bootable installer, choose ‘Disk Utility’ from the available menu.

Then select the desired external drive from the startup manager screen and press the enter key.

If you have an external bootable disk, connect this to your Mac and power your Mac up whilst holding down the OPTION/ALT key.

Some of the following steps are destructive and will lose ALL data on your drive. I would strongly recommend at this stage attempting to back up any data that is required before proceeding with the following steps. To create your own bootable disk, refer to our blog ‘Creating a Mavericks bootable install disk’. You can then run Disk Utility from there.Īn external drive or a network drive is preferred if it is the internal hard drive you have an issue with, since the OS X Recovery is a partition on the same physical drive which may not be able to successfully unmount or modify your internal disk. 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.

Failed to erase disk mac