How to keep copy of sub-set of images on a separate drive?

Smart previews are indeed a great solution for this. However if you want the full resolution files on another disk, the way to do this is to first create a folder on the other hard drive (SSD or whatever) and make sure it shows up in Classic. The quickest way to do that is to put a dummy image in there and drag that image on Classic. Import it using the "add" setting. This will make the folder show up. Now go to your collection and select all images (you can also try it with a smaller subset first). Drag those images into the folder you just set up in the sidebar. Wait until they are moved and the images are now in the new location and still in your collection. This won't work if your images are virtual copies and the originals are not in the same collection though.

That's what smart previews are for. If you generate smart previews for those images, you will be able to edit them while the originals are offline.