Importing photographs into your catalog establishes a link between the catalog and the physical file. The photograph can exist on an internal or external drive. In previous versions of Lightroom Classic, you could edit images that were contained on drives connected to Lightroom Classic.
Smart Previews in Lightroom Classic allow you to edit images that are not physically connected to your computer. Smart Preview files are a lightweight, smaller, file format, based on the lossy DNG file format.
There are several ways to generate Smart Preview files:
When you import new images into your catalog, select Build Smart Previews (Import dialog > File Handling section). Smart Previews are created for all the images imported into the catalog.
When you export a set of photographs as a catalog, you can choose to build and include Smart Previews in the exported catalog. Click File > Export as Catalog, and then select the Build/Include Smart Previews checkbox.
On the fly
You can create Smart Preview files on demand. Select the files for which you want Smart Previews, and then click Library > Previews > Build Smart Previews.
Smart Previews are stored in the [Catalog Name] Smart Previews.lrdata file, located in the same folder as the catalog.
You can identify the Smart Preview status of a photograph based on the information displayed below the Histogram:
Do one of the following: