Also, if your photos are managed (located within the Aperture Library) to export them out to an external HD or another folder on your computer's HD that is NOT located within the Aperture Library, you have to select all of them them (the projects), click File>Relocate Originals for Project, then select which folder in Finder you want them to reside in so that Aperture knows where they are. These photos now become referenced. I think that's what you're asking.