Bodine - I'm not clear what you're asking. Some media players have "move" functions - I don't know if WMP does or not. If you have the same music file in two different places on the same computer, WMP (or any other player) will see them as two different files if you Import Media. You have to delete the file from the drive AND delete the entry from your WMP library to get rid of it.
I don't much care for WMP for several reasons, so I'm not completely familiar with it. Another way to get rid of the internal drive files on many players would be to filter by physical location (in this case, your internal drive) and delete all those files from within the player. This will delete the physical files and the player db entries.
It is definitely possible to drive a player from an external HD only.