Today’s Question: When I choose to build previews [in Lightroom Classic] and have XMP sidecars are the previews in the XMP?
Tim’s Quick Answer: No, previews are stored alongside the catalog in Lightroom Classic. The XMP sidecar files only contain metadata updates.
More Detail: If you save metadata to raw captures in Lightroom Classic, metadata updates are saved to XMP sidecar files in addition to being saved in the catalog. You can save metadata to selected images manually by choosing Metadata > Save Metadata to File from the menu. You can also enable the automatic saving of metadata to the source files by turning on the “Automatically write changes into XMP” checkbox on the Metadata tab of the Catalog Settings dialog.
However, saving metadata to the source files does not cause previews to be saved to those files. Previews are only saved alongside the catalog, in a previews file in the same folder as the primary catalog file. This enables you to view previews of your images whenever you open the catalog, even if the source image files are not currently available, such as when an external hard drive containing images is disconnected from the computer.
Note that when you back up the Lightroom Classic catalog, the previews file is not backed up. That means that if you needed to restore from a backup because the catalog had been lost or corrupted, you may need to rebuild all previews again unless you had also backed up the previews file separate from the Lightroom Classic catalog backup.
