Opening the catalog from the backup hard drive is still going to look for the photos in the original location. And if that original location is an external hard drive that isn't currently connected then the photos aren't going to be available in Lightroom. The catalog does not contain the images. It contains a pointer to where the image is stored on your computer, as well as a history of adjustments that are made to that image. Lightroom always looks in the same place for your images. So if those images are on an external hard drive then that drive must be connected in order for Lightroom to function properly.
Thanks for the response, Jim. That explains what is happening. However, I'm also (perhaps more) interested in whether Lightroom can be easily redirected.
Perhaps there is some confusion over my use of "catalog". When I drag and drop my "Lightroom 5 Catalog" folder onto my backup drive, it includes my image files in the same file structure as on my original drive. So, my backup drive now includes not only the .lrcat and .lrdata files, but copies of all my image files, as well. The question, I guess, is whether there is an easy way to have my Lightroom catalog on my backup redirected to look for my image files on the backup drive.
Your response also raises the question of whether my Time Machine backup will suffer from the same issue.
When referring to the "catalog" in Lightroom, we are usually talking about the Lightroom catalog which is a database. It keeps track of where the images are located on your computer as well as any adjustments that are made to those images. In my opinion, it is a good idea to always allow your backup Lightroom catalog files point to the original images that you are working on using Lightroom. Otherwise, you can have several different versions of the same image. The idea of the image backup is to have something to fall back on in case of a disk or computer crash.
If you really want to redirect Lightroom to the backup images then it would be a good idea if all of your images were in a folder structure that puts all of your image folders within one master image folder. Then, in the backup catalog, you could right click on that master folder and choose the option to "update folder location" and you would be able to point Lightroom to the backup files. By doing this on the master image folder, it will update the location for all subfolders and images. But as I indicated, I think it's a better idea to leave the backup Lightroom catalogs pointing to the images in their original location. Otherwise, if it becomes necessary for you to use a backup catalog because of catalog corruption, the backup catalog will be pointing to the backup images which might not have all the adjustments.
A couple of examples might help explain a little better. Let's assume first that you encounter a corrupted catalog. All that's necessary is for you to replace that corrupted catalog with your most recent backup. If the catalog is still pointing to the original images then you can simply resume your work. Of course you will have lost any changes that were made since that backup was created. So let that be an indication of how often you decide you want to backup your catalog.
Now let's say that your catalog is okay, but your images are on a hard drive that crashes. Then all you have to do is point the catalog to the backup images and all of the adjustments in the catalog will be applied and work can continue uninterrupted. You might want to copy those backup images to another new working hard drive. If the hard drive has the same name or drive letter as it did previously you will just continue working. If to drive letter changes all you will have to do is update the folder location. All of the adjustments will still be in place and work continues.
Thanks! I think that does it.