Ok if you click on the little arrow button at the top on the right near the content tab you get a menu with one item that says search for content.   When you select that you should get a dialog box listing all of your drives and you can select or deselect them there.  I did this while having "view folders as list/tree" selected instead of "view categories" but another option could be to switch between category view and folder view then switch back.

Just some thoughts.  Also, does studio see the content in your external drive when viewed from folder view?

Hope this helps some.