I have an external HD (G-Tech) that I have some of my older photos stored as referenced files to free up space on my MBP HD for my remaining managed files. As swordio pointed out, you can always move the files from your HD (managed) to the external HD (referenced) at any time. Or you could import the photos from your memory card directly into your external HD. When I click on the import button, my MBP HD shows up in the upper left hand panel in the "Inspector" portion. However, in the middle section toward the bottom BOTH my MBP's HD and the external HD show up in Finder-like fashion. Clicking on the external HD reveals all of my files of which I normally keep back ups of all of my RAW photos as well. I have my referenced Aperture photos arranged in "libraries" by grouped years to make it easier to find a photo when searching through 1000s. So I am not sure why you are having such a problem. BTW, and FWIW, when I do an import from my memory card to Aperture as a managed file, I select from the drop down menu of Import Settings>Backup location. I pick "choose", find my location on my external HD (my file is named 2014 RAW) and then also select Project name in the subfolder section. Aperture will then automatically copy the RAW files from my memory card to my external HD as a back up.