I back up LR often and like to keep a copy of the current "main" LR backup folder (the one containing previews and helper data in addition to the catalog) on two identical external backup drives. I have 175,000 image files and it takes a while to copy the folder because of the previews.
So the question is: When I drag and drop to copy the "main" folder to a backup drive, the Mac OS asks if I want to replace the existing folder with the new one or merge it with the newer one. (See attachmnet.) I've never been sure if merge is the best choice, as there will have been both additions and deletions to the catalog and previews, and I want to be sure the older items are deleted.
(I realize the previews can be recreated as necesary, so maybe I really don't need to take the time to back them up and should just copy the catalog file.)
The only file I backup is the catalog file, and I use the backup feature that is built into LrC. That process creates a complete backup of the catalog as it exists at the time of the backup, and creates it as a separate backup file each time. The previews can be regenerated anytime, as can the other file. So I have never found a need to create backups of those files. I wouldn't attempt to advise you on the best approach to the way you have been doing it. Personally, I think creating a new and separate backup of the catalog is the best approach. What you choose to do about the other files is up to you.
Your primary backups of the catalog should be on a separate drive from the main catalog. This is one of the large mistakes Adobe makes when setting up a new catalog.
There is no need to copy the preview folders, nor the main LR folder. When manually copying the "backup" folder tree to a new drive, the best option for existing files already in the destination folder is to "skip", not overwrite, definitely not "merge".
Make sure when you do all this copying that LR is not running and has gracefully shut down. Your attached screenshot suggests it was not.
Thanks Joe! I started a "dummy" copy to generate the screenshot -- I do shut down before backing up. Special thanks for the red flag on "Merge" -- that's what I thought, and have never done it, but I wasn't sure. I'll simplify things and just back up the folder with the .zip catalog file.