Today’s Question: My preview file [in Lightroom Classic CC] has grown to a huge size because I use the full-size preview option for faster browsing. I am running out of space on my laptop and have in the past trashed the file and started from scratch. But it takes a long time to rebuild the previews. Is it possible to move the preview file to an external drive with larger capacity?
Tim’s Quick Answer: It is only possible to store the previews file on an external hard drive if you also move the Lightroom catalog file to an external hard drive, since the catalog and previews must be in the same location. Otherwise your only option is to discard 1:1 previews, or to delete the previews file and have Lightroom re-build previews as needed.
More Detail: Lightroom creates previews for your images so they can be browsed more quickly, and so you can view your images even if the source file is not currently available. The previews are stored in a file in the same location as the catalog file that contains all of the information about the photos you’re managing in Lightroom.
While the catalog file will be relatively small even when you are managing a large number of photos, the previews file can get quite large. For example, my previews file has grown to over 100 gigabytes for my catalog of about 400,000 photos. That means the previews file can consume a considerable amount of storage space on your hard drive.
If you delete the previews file you won’t actually lose any information, as Lightroom will simply build the previews again the next time you browse your photos. Of course, that means browsing will be a slower experience until all previews are generated. The previews file will be in the same folder as your Lightroom catalog, and will have the same base filename as the catalog but with “Previews” added to it. The filename extension is “lrdata” rather than the “lrcat” extension for the catalog file.
In the case of 1:1 previews, you can actually discard those without deleting the previews file. Start by navigating to the folder location you want to delete previews for (or the “All Photographs” collection in the Catalog section of the left panel in the Library module. You can select the specific photos you want to discard previews for, and then go to the menu and choose Library > Previews > Discard 1:1 Previews. The confirmation dialog will give you the option to discard previews for only the selected photos, or all photos in the current location. Standard previews cannot be discarded in this way, but of course 1:1 previews are generally larger and therefore have the potential to consume more storage space.