I keep 90% of my photos on external drives, and because of this I got in the habit of creating smart previews as part of the Import process. All you need to do is check the Build Smart Previews box in the File Handling panel of the Import window to add that into your import process. By having smart previews for those photos I can continue to work on them in Develop, export or email a small JPG, do a quick HDR or pano merge, or just basically continue my workflow for the most part with or without the external drives connected. Over time, those smart previews do build up in the special preview cache...