Moving LR Classic catalog from MacBook Pro internal hard drive to external

For everything to work, two things must be available on a mounted volume:

• A catalog’s folder (containing a catalog and its associated .lrdata files). 
• The images and videos that the catalog entries refer to must be at the locations the catalog recorded when they were imported. 

So far it sounds like you’ve been talking about the catalog. But what about the images and videos listed in the catalog? 

You’ll know everything works if:

• You can open the catalog you want. 
• In the open catalog, no folders or files are marked as missing (you don’t see any “?” icons). 

I thought I read that the next time I want to open LR I will click on on of the files on the external and somehow LR will then do that automatically in the future.   But does that also mean I won't be able to use LR without the external drive being connected?

By @Reilly31479077qcuf

This is confusing. You were talking about moving a bunch of files to the external volume, but now you’re concerned about needing the external volume.

The straight technical answer to your question is that yes, you can use Lightroom Classic without any external volumes…as long as you don’t need anything on those external volumes. For example:

• You want to start Lightroom Classic. The application itself is normally installed in the Applications folder on your MacBook, so that works fine when no external volumes are mounted. 
• You want to open a catalog on your MacBook or start a new one to store on your MacBook. Of course you won’t need external volumes for that because it’s all on the MacBook. 
• You want to open a catalog you’re storing on an unmounted external volume. Sorry, now you have to mount that volume to get at that catalog. 
• You open a catalog, and the photos and videos in it are stored on an unmounted external volume. As long as the volume containing the originals is unmounted, you can only work with the thumbnails and you can only enlarge images if they had previews that were already generated. The Develop module will not be available for any originals that are on an unmounted volume; you’ll have to mount it to edit in Develop. (Smart Previews are a workaround for this.) Once you mount the external volume containing the linked originals, everything will work again.

In other words, when you say “able to use LR” it’s important to make the distinction between Lightroom Classic itself, any of the catalogs you might create and where they might be stored, and any of the originals referred to any of the catalogs and where those are stored.

If the catalog of interest is on the external drive, then the external drive must be connected in order for you to use that catalog.

Make sure you have everything working from the external drive before you start deleting anything. It sounds like (you didn't make this clear) you have multiple catalog files, so you will need to make sure all of them are working from the external drive.