Today’s Question: I recently had a huge problem with Windows 10 and had to restore from a backup. Immediately afterward Lightroom would not launch, saying the catalog was corrupted. I finally got it working by making a copy of a backup catalog. I was under the impression that thumbnails for files on an external drive that is not connected would still show up in the Library module. Now I get the exclamation point [indicating photos are missing] but the thumbnail is just gray. What am I missing here?
Tim’s Quick Answer: You’ll likely need to re-create the thumbnails and previews for your images, because it appears the file that contains the previews for the photos in your catalog became disconnected from the actual catalog file.
More Detail: Because Lightroom employs a catalog to manage the information about your photos, you are able to browse images even when the source files are not available (such as when the external hard drive containing photos is disconnected from your computer). This can be a tremendous workflow advantage, of course.
However, the previews for your photos are actually stored in a file separate from the catalog file. Furthermore, when you backup your Lightroom catalog, the previews are not included as part of that backup.
When you restored from a backup copy of your catalog, the previews file from your original catalog may not have matched the backup catalog you restored from. Therefore, you would need to build previews again, which will also provide you with updated thumbnails for all photos.
To build previews for all photos, first make sure the source image files are available to Lightroom, such as by connecting the external hard drive that contains your photos. You can then choose “All Photographs” from the Catalog section of the left panel in the Library module within Lightroom to view all images. Next, select all photos by choosing Edit > Select All from the menu. Finally, choose Library > Previews > Build Standard-Sized Previews from the menu. Lightroom will generate previews (and thumbnails) for all photos in your catalog. This process will require a bit of time, but once the previews have been generated you will once again be able to browse your photos even when the source image files are not currently available.