Steam making me reinstall

I keep aoe 4 on a external drive I plug into my laptop. Went to start it up and all the games on the external drive would not show up installed. Clicked on something about steam library drive D and the other games all got their play buttons back. And they give me the play buttons even with my external drive is not plugged in. But aoe 4 is making me reinstall the game 27 GB all over again. And somehow the game files disappeared on the external drive.