In some situations, you may need to merge two separate Lightroom catalogues. Let’s say you have a catalog stored on your computer, and one on your external hard drive. Here is how you can quickly merge the two, and create a new catalog that includes all of the photos you would like to merge into one master catalog:
The first catalog I want to open is titled “1.” Open your first catalog in Lightroom by double-clicking (PC) or dragging it into Lightroom (Mac).
Once you have the first catalog open in Lightroom, select File>Import from Another Catalog, then click Import with “Add new photos to catalog without moving selected” – this will bring your images into Lightroom without changing their location on your hard drive.
You now have both catalogs in Lightroom, and you can see all of them by clicking “All Photographs” in the upper left side of Lightroom.
To export both catalogs as a new, single catalog, select File>Export as Catalog. You will then be prompted to name your new catalog, and select its destination on your computer/hard drive.