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.
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
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ps ax | grep fsck_hfs |
You’ll get something like
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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
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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”.