Teachergirl1963 wrote: backed up to an external drive. … I would like to be able to store all my old Photos also in the iCloud in case some terrible accident occurs to my Mac book.
"also" is good. iCloud is only a limited backup, since it synchronizes mistakes as quickly and completely as it copies pictures.
I have had a Quick Look through the website and It appears to say the second Library cannot sync to Icloud? Is that correct? I quite liked having my old photos in a second library as it didn’t clutter up my phone and stuff.
I have many Photos Libraries on my Mac.. I also have more on external drives. I have my wife's old family pictures in one Library, and I have my own family's old pictures on another. I have a Library just for my Nikon's pictures. And I have a "Favorites" Library that is my System Library that is connected to iCloud and synchronized with my phone and iPad. A Mac can have many Libraries, but iDevices can have only one. Perhaps " second Library cannot sync to Icloud" means that they can't both be connected to iCloud, and that's true.
Here's a link to a guide to making a new Library:
Make a New System Library connected to iCloud - Apple Community
You don't need all of that, but you will need to designate your "Favorites" Library as the System Library.
I keep my smaller "Favorites" Library on my MacBook rather than on an external drive. With a portable MacBook, I can't keep an external drive dangling as I move around. I do have other Libraries on a small, 1 oz SSD that I carry with me. But the System Library needs to not be plugged and un-plugged all the time-- that gets very confusing to iCloud.
Most of us started by duplicating the big Library and then removing pictures-- that's easier than selecting pictures and copying them to another Library.
An important part of having multiple Libraries is using the trusted 3rd party app PowerPhotos ($40.) PowerPhotos makes it easy to copy pictures from one Library to another, and copies can maintain our Albums and Folders organization. I think pretty much everyone who maintains multiple Libraries uses PowerPhotos.
So there's lots of experience here. Don't hesitate to ask…