To move the catalog (and associated previews in the folder whose name ends with Previews.LRDATA), you simply go to your operating system, move these files to wherever you want, and then double-click on the catalog file to open it in Lightroom.
Photos should not go on the SSD anyway, you would import them to some other drive; when you do the import, select whatever location you desire in the Destination Panel.
Not quite what I'm asking, though. In order to move the Library files I first have to create them. When I create the Library, the default location of the import is to the Mac's Pictures folder on the main (SSD) drive. What I'm trying to do is change the behavior so that the default location for creating the Library is on the other drive. The way it is currently defaulting is rendering it impossible for me to import my photos into Lightroom. I will run out of room after I import the next 20%.
and, you're right, the photos shouldn't be on the SSD. The photos that I'm importing are on an external drive.
Lightroom is going to create those files the first time it opens. And it's going to put them in the default location. Let Lightroom do it's thing. Then, before you do any work at all in Lightroom, Shut down Lightroom and move the folder that contains all those files to wherever you want them. Double-click on the catalog (.lrcat) to start Lightroom. The default preference setting for the catalog is to open the last one used. So now, the catalog in that new location will be the one that will open.
Aha! That makes a lot of sense. I will move that entire folder over to the other drive and do that.
Thank you both for your help!!