Export as catalog

The command can be difficult to find because it isn’t on the menu bar, only on a context menu. To do this, right-click the folder and choose Export This Folder As a Catalog. If right-click isn’t enabled on your Mac for the trackpad/mouse/stylus you’re using, Control-click the folder to see the menu. 

But make sure this is something you really want to do, because Lightroom Classic makes it kind of a pain to manage multiple catalogs. Make sure you actually want to split off those images from the catalog. If the goal is to free up storage space on the current volume, you don’t really need to make a new catalog. Instead, you can simply move the folder to the other hard drive and remap that folder’s location in the same catalog, because a single catalog can track photos and videos even if they’re spread across multiple volumes.

You can make a new Catalog that contains only those selected photos. This new Catalog can live wherever you want. The photos will still be seen in the current Catalog too - unless you remove them from that. 

The underlying source files (camera Raw, camera JPG, Photoshop edited files etc) that are referenced by this new Catalog can either be the same ones in the same locations that the current Catalog is referencing - so that both would share in using these - or else you can instruct copies of just the relevant files to be made alongside this new Catalog, and those would be the ones it would then reference. Again, whether you instruct this or not, the starting files and folders would all still remain in place regardless - unless and until you deleted those from disk. 

In other words  you can either selectively and partially copy your Catalog to another disk, or else you can do that and at the same time copy all the relevant source files for the photos involved.

This function is called "Export as Catalog".  

Well sure, but there's really no need to put part of your catalog on a different HDD. Better just to move the photos to a new hard drive, while keeping them in the same catalog as now. The photos take up more space than the catalog does. And one catalog is usually better than several catalogs.

https://www.lightroomqueen.com/move-photos-another-hard-drive-leaving-catalog/