I wanted to use an USB3 External Hard Drive to a Virtual Machine on my server which was running Windows Hyper-V 2016 Server. From searching on the internet it seemed you needed to make the Hard Drive offline, however not much information on how to actually do this. In the end this is my solution:-
Exit to Command Line from Hyper-V server 
Then enter disk management called diskpart [diskpart]:-
List the volumes [list volume]:-
Find the one that is the Hard Drive you wish to make offline. In this case it is Toshiba Ext, Volume 4.
So select the correct volume [select volume 4]:-
I like the make sure it is actually selected as I don't want to down my server.
So list the volumes again [list volume]. Notice the * next to Volume 4 indicating that it is selected:-
Make the disk offline [offline disk]:-
After this in Hyper-V manager you can now create a new Hard Drive selecting an actual Physical Hard Disk, in this case my Toshiba USB3 HD:-