The 2.5" drive gets the 5V power thru the flat ribbons four extra wires,
but an 3.5" drive needs 5V AND 12V PSU to the separate connector.
Best thing should be to use a PSU from a peesee. That PSU can feed
the Falc as well; then you can remove the original PSU, and place the
3.5" drive inside your Falc. :)