volume could not be unmounted disk utility

This is the recommended method because it should always fix the error. How to Resolve the Unmount Error with a USB Boot Drive We’ll also show you a way to forcibly unmount a disk by command line, though that approach must be used with caution as it can lead to data loss on the drive in question. We’ll cover both with a bit of explanation. This will allow you to fix the problem, regardless of the cause, by one of two means, the first is a sure-thing to fix the issue, while the other only works sometimes.

For the boot drive, it shouldn’t matter which version of Mac OS X it’s for (assuming 10.7, 10.8, 10.9, 10.10, 10.12, 10.13, 10.14, etc at least), the only requirement is that it has Disk Utility – which they all do. For the former situation where the boot drive is being modified, the easiest solution is to boot from another drive and run Disk Utility from there instead.

Typically the “Couldn’t Unmount Disk” error pops up when the currently boot drive is being modified, or if a disk was trying to be erased you may find the erase failed with a couldn’t unmount disk error.

Volume could not be unmounted disk utility