There is no "best" method, there are methods that will work for you and methods that will not work for you.
If the folders are on the external drive, then "Add" certainly would work, it leaves the photos in the existing folders.
In almost all situations, you want to use a single catalog. (The only situations where you might want multiple catalogs are if the subject matter of catalog one is completely non-overlapping with the subject matter of catalog 2, for example, business and personal).
You can combine catalogs by opening one, and then use File->Import from Another Catalog and point to the other catalog.
Thank you very much. I had read or heard that I should copy or move rather than add. Didn't make sense, which is why I asked.
Your response was very timely and to the point. Thanks again, I will do as you recommended.
Enjoy the holidays!
Another quick question:
I have a lot of folders in my external drive and when I click on one to import and I get a blank display where the grid would normally be, does than mean that those images are already somewhere else in LR catalog? I notice the "Don't Import Suspected Duplicates" is enabled.
Other selected folders clearly indicate "No photos found" which means I put them elsewhere or deleted the images without deleting the folder, correct?
On a very few folders, the grid is filled with "Preview unavailable for this file." I am not sure what that means.