Migrating photos from one catalog to another

[HSakols]

On an older external hard drive I have rediscovered a catalog that contains a number of photos that are missing from my present main catalog. I'm thinking that I will just go to my main catalog and import the old catalog and associated photos which amount to about 22,000. I just need to make sure that I don't import duplicates.  Would this be the best way to ensure that I have everything from my old catalog?  Any advice before I proceed would be appreciated.  The old drive is usb 2 so I imagine the process will take some time.   

[JoachimStrobel]

In the to be imported catalogs, I did move all my collections into a collection group starting with Z-.
So then they are tidy at one place amongst all the other collections in you big catalog. And you can move them as a group. Obviously, you want to move the originals to a new place upon importing the catalog.

[HSakols]

Thank you.