Remove Lightroom Contents from Mac Hard Disk and Transfer to Hard Drive

Exporting is not a step here. (And exporting will not cause you to lose photos anyway).

You COPY the entire folder hierarchy of photos USING YOUR OPERATING SYSTEM, without changing their position in folders and without changing the hierarchy of your folders, to a new disk. Then using your Lightroom Classic catalog, you reconnect (see Part 2 — Updating Folder Location here), and then once you are sure everything is working, you can delete the copies of the Mac hard disk.

Although you're no moving to a new computer, some parts of this guide may still be helpful in understanding the process of moving the catalog and relevant data.

https://www.lightroomqueen.com/how-move-lightroom-to-new-computer/

1. Your photos are NOT in the catalog. They exist in folders on a drive on your computer where Lightroom Classic (LrC) placed them on "Import". You have total control of where LrC places your photos files. You need to know exactly where those photo folders and files are so you can make regular backups.

2. The LrC Catalog and it's associated files, including the previews it makes are in their own folder and subfolders. The LrC Catalog is a file with the extension .lrcat. Here is a link to an Adobe help article explaining how to find your catalog.

It is entirely possible to relocate the entire Catalog folder, either by copying it to a different location. You really need to know what you are doing to accomplish this. The catalog is the heart of every thing LrC does and contains all the information about the adjustments you have made to your images. It is essential to back it up to a drive that is different than the one where it is located for everyday use. Also, the LrC catalog should be run from the VERY fastest drive your computer has for best performance.

3. Because the photo image files are not part of the LrC catalog, they can be located on any drive you have available. Moving the image/photo files is a better way to save space on your drive than moving the catalog. Depending on your computer, you can add additional drives, internal or external. Be sure to have a different drive you use for backup.

4. When you move your image files, be sure to move them in a way that their organizational structure stays identical to the way they are now. Once the files have been relocated, you will have to tell LrC where they are. It's also possible to use LrC to move the files so it doesn't loose track of where they are once the new drive is available.