Plug in a disk with a bootable install of macOS on it. On older Macs that don’t have a T2 security chip, boot from an external disk like this: You should now see macOS Utilities from where you can choose Disk Utility and then unmount the volume from there. Type in your username and password if asked. Power on your Mac and keep holding the power button until you see ‘loading startup options’. Release Command-R when you see the Apple logo. Hold down Command-R and power your Mac on. So, on those Macs, you should boot from the recovery partition by doing the following: You can turn it off using Startup Security Utility, but that’s beyond the scope of this article. These Macs have secure boot enabled by default, which prevents them from booting from an external disk. However, on Macs with T2 security chips (effectively any Intel Mac from the 2018 MacBook Airs, MacBook Pros and Mac minis onwards), and Macs with Apple silicon, it’s not that simple. On older Macs, that means booting from an external disk or USB stick. The only solution is to boot from a different volume than the one you’re trying to modify. There is always a risk of losing data, and making a full backup is the only way to protect your data. 
#Couldn t unmount disk external hard drive how to#
How to fix ‘couldn’t unmount disk’ errorīefore you try and fix any problem with a disk, including one that can’t be unmounted, you should back it up. Essentially, the erase process has failed on your Mac. For example, if you try to erase or partition your boot disk in Disk Utility, you won’t be able to because macOS won’t unmount the disk. If you see an error message telling you that macOS couldn’t unmount a disk, it usually means that a process on your Mac is trying to modify your boot disk and can’t because macOS won’t allow the disk that it has booted from to be unmounted.
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