We need to be careful about the words that get used here!
Your images are not "in" the Lightroom catalog. Your images are on your hard drive(s) where you originally put them (or had LR put them), just like your spreadsheets and word processing documents and your MP3 music files.
The LR catalog is a database file. It contains references to the image files using the files' path and file names. The catalog (*.LrCat) is a relatively small file, probably less than 1 gig.
Associated with the catalog, and in the same folder, are two folder trees, one for the standard preview files and one for the smart preview files. These folders will probably take up a goodly amount of space, but nowhere near the space that your original images take up.
If you don't need the smart previews at all, turning them off in LR and deleting them from the hard drive is the easiest thing to do.
If you do need some of them, you can put expiration dates on the smart previews.
If you need more space than that, you can move your original images to a different drive (internal or external) then just tell LR where you moved them to. This is the most common method of freeing up internal hard drive space
If you delete your original images from the hard drive, they are gone forever, unless you (not LR) created backup copies.
If you remove the images from the LR catalog, you will lose all the edit information and their metadata.
"If you remove the images from the LR catalog, you will lose all the edit information and their metadata."
Does that include the smart previews? That's my question.
I use the smart previews during editing & I don't need it taking up space after that. How do I set expiration dates to my smart previews as you mentioned?
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In the Catalog Preferences dialog, File Handling tab, there is the "automatically discard 1:1 previews" setting. However, I am now not sure if this also governs the smart previews.
If you remove images from the catalog, yes, that will also delete your smart previews for them. However, this is a drastic approach for just wanting to delete the smart previews.
To just delete the unneeded smart previews from some of your images, in the Library module, using folders or collections or filters, select a bunch of images, then go to the menu /Library/previews/discard smart previews/