Smart Preview (Adobe)

With Lightroom‘s Smart Previews you can store smaller, lower-resolution versions of your photos within the Lightroom catalog while storing the full resolution versions on an external disk drive. Smart Previews are compressed DNG files and fully editable – any changes you make are automatically used for the full resolution photo when your drive is reconnected. Smart Previews make it practical to view and edit your image library on a laptop with a relatively small internal drive.