Hello
I had a similar problem, after forcing - shutting down the computer, the external drive did not show up any more on my desktop. Running diskutil list in terminal did show the external drive but I could not unmount it, message error was unable to unmount the drive. Running diskwarrior was not a solution either because the message was, disk not mounted.
What I finally did, was to shut down the computer without extracting the external HD, then plugging the device in a windows computer, when the computer recognizes the drive and you are able to search into the drive, then turn off the computer without extracting the drive. Extract the drive after the computer is switched off.
Turn on the mac without plugging the drive, when desktop is available, plug in the drive again.
It seems the drive stayed hanging in I don´t know what.
I hope it works for you guys.
Regards