A very common mistake new Lightroom users make is to create multiple Lightroom catalogs. I've even helped users who have created a new catalog every time they imported, with one of them having over 200 catalogs! Yikes! Most Lightroom users only need to have one catalog. Even if you are a photographer, it's highly advised to keep just one catalog, and use collections to keep your personal and professional images separated.
What if you already have made multiple catalogs? Don't worry, there is a fairly simple fix. All you do is "Import from Another Catalog," for each alternate catalog you have. I actually had three Lightroom catalogs: one for my Mac mini desktop computer, an old one from a PC desktop that bit the dust two years ago, and my laptop catalog. I recently imported the Mac Mini and the PC catalogs into my laptop catalog, and I thought I'd share the process with you.
Importing from Another Catalog
I know I talked about "merging" catalogs, but in actuality you will be "importing" pictures from another catalog. When you do that, all the edits, keywords, ratings, flags, collections, everything attached to each and every image in that catalog gets brought into the catalog you have open. So, you are "merging" that particular catalog with the one you are using at the time.
You start this by opening the catalog you want to have as your 'master' catalog. Then go to File in the top menu, then down to 'Import from Another Catalog.'
Find the Catalog You Want to Merge
When you click on that option, you Mac Finder or Windows folders will open up. You must navigate to wherever the other catalog is that you want to merge. Below you can see that I went to an external drive where I had copied the catalog from my desktop computer.
Click on the file that ends in .lrcat, and select "Choose."
Options to Work Through
Once you click 'Choose,' you will see a menu like the image below. Slowly go through each option, making sure you are happy with the selections. You can choose which folders you want to import from, if you want to move the photos or simply add them to the catalog, or if you want to replace images that you may already have in the current catalog.
After I imported images from both catalogs (Mac desktop and PC desktop), I had a lot of housekeeping (file management) to tackle. I moved folders from one drive to another, as well as finding and eliminating duplicate photos. Make sure you do all of that while you are INSIDE Lightroom.
*** Please note that the items in the green boxes are NOT how you should have them, rather highlighting areas that you need to make choices.
Once you have all the images imported (that you want), you can simply delete that old catalog, and the support files that come with it. Or, you can store it away on a separate drive. After you have all your various catalogs imported, if you like you can rename the master catalog to something memorable, so you know for certain that is the catalog you want to use each time.
The feature image (flower) was a one-click edit, using Calypso from the Film Bohemian Collection.