When you are ready to use the files on hard drive (B), the proper way is to move them back to the exact same locations the originals were stored in on disk (A), and then open Lightroom.
If you absolutely must use the files still on drive (B), you'd need to reconnect all of the photos.
By the way, you didn't mention this so I mention this: you also need to make regular and automated backups of your catalog file; backing up the photos is not sufficient.
I want to take drive A to my safe deposit box, so I will not be using it any more. So I must use Drive B. The files on Drive B are in one master folder. How do I reconnect the master folder to the Catalogue.
I backup the Catalogue every time I close LR, and back it up to three different external hard drives. (Yes, I am paranoid!)
Thanks for your help.
Thank you for your help, that link was just what I needed!
Still at a loss as how to proceed. Let me explain further: I had 2 external hard drives both with exactly the same photos, because when I imported a photo into LR I also made a second copy to the other drive. Each of my drives contained 135,000 images, but my catalogue said it contained 270,000 images. So I had a situation where any changes I made to an image in one hard drive would be shown if I opened the corresponding image in the other hard drive.
One of the drives failed and is gone. I still have 135,000 images on my one remaining drive, but my catalogue now says that there are 135,000 images in it. I want to be able to set up the second external hard drive so that all changes that I make to an image on one of the hard drives will be reflected in the other hard drive also.
The link you sent me does not discuss this issue. Thanks in advance for your help.
I want to be able to set up the second external hard drive so that all changes that I make to an image on one of the hard drives will be reflected in the other hard drive also.
This is not a capability of Lightroom. Lightroom does not do this. And this indicates you do not understand how Lightroom works. The changes are stored in the catalog file, not in the photos. You really want your originals on both disks, not the changes.
So a more realistic expectation, that meets the way Lightroom works, is to have identical sets of photos on two disks, and the changes arer stored in the Lightroom catalog (with appropriate backups), and if you need to use the photos on a different disk, you can use the directions in that article to point to the photos on the different disk.
If your old setup did as you describe, you had something else going on in addition to Lightroom.
Your images are not "in" the Lightroom catalog. They just exist on the hard drive(s) and are referenced by LR using the full path and file name.
Lightroom does not directly edit your images. All edit commands are stored in the catalog and played back for you to see the results. If you want to see the results outside of LR, you need to "export" your edits to a new file (JPG or TIF).
Your master images exist in only one place, even if you have LR make backups on a second drive during the import. LR will never see the second copy of the files.