OSX Disk Utility Erasing/Formatting stuck at "Unmounting disk ...", "Couldn't unmount disk"

If you are trying to format/erase a disk and disk utility/diskUtil is stuck at "unmounting disk", probably "fsck_hfs" is stuck in a loop, trying to repair it before mounting.

couldnt-unmount-disk-disk-utility-error

I've seen solutions where you must boot from an external operating system, bla blah. Too complicated and time consuming.

To fix this fast and easy, start formatting, wait for "unmounting disk" message to appear, then go to Terminal, find the process with the following command
ps ax | grep fsck_hfs

You'll get something like
simion@Simions-MacBook-Pro:~$ ps ax | grep fsck_hfs
3693 ?? U 1:32.61 /System/Library/Filesystems/hfs.fs/Contents/Resources/./fsck_hfs -y /dev/disk2s2
3984 s000 S+ 0:00.00 grep fsck_hfs

Then ... kill the process with
sudo kill 3693

Then it should continue automatically with formatting the drive.

Note that you must do this before disk utility gives the error message "Could not unmount disk".

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