Some suggestions:
Assign a drive letter to the Ext drive like P,Q or something so it doesnt get changed evry time you plug in a thumb drive or something.
NEVER disconnect it without using the "safe to disconnect". (this is the only time I have lost data from yanking out a usb connection). You may corrupt the MC database.
Move your media to the Ext drive set up auto import rules to see the media folders on the ext drive.More advanced stuff
I set up a new library with the MC database and all assoc files residing on the drive where I can easily back it up and not have it mixed in with operating system files
you can even shift your existing library there......The default library is called Main Library.It is loacated at...AppData\Roaming\JRiver\Media Center 18\Library
DLNA Library is also here.
It is not possible (MC18) to Move the main library location. It is not possible to delete the Main library.
Workaround
In Library manager clone the existing library to ext drive location .