disk utility first aid failed external hard drive

Now perform the original task that threw the “Couldn’t Unmount” error.Go to “First Aid” and verify the disk, then repair if needed.At the boot menu, choose “Disk Utility” (if using an Installer disk, pull down the “Utilities” menu to access Disk Utility).Hold down the OPTION key during boot, then select the attached boot drive (typically has an orange icon at the boot menu).Attach the USB boot drive to the Mac and reboot.You will need any Mac OS X boot drive to complete this task, I used a Mavericks boot installer drive for this purpose but others should work too, whether they are installation drives or just recovery drives, the important thing is they are bootable and separate from the primary boot disk that stores the installed OS: 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.

Disk utility first aid failed external hard drive