How can I copy pictures in Lightroom from one external hard drive to a new hard drive? When I try to import/“copy” images from the original hard drive, virtually all are grayed out, even though I've unchecked the 'do not import suspected duplicates' box, and I can't 'copy' or 'add' the images to a newly created folder on a new drive.
You can't import files to Lightroom Classic (or earlier versions) using the actual files which have already been cataloged, which is what you are trying to do. The "Do Not Import Suspected Duplicates" option doesn't apply in this case, as the images you are trying to import are not "duplicates", they are the already imported files.
If for some reason you want to have two different copies of the same file in the Lightroom, you would first need to copy them from the existing hard drive location to the new hard drive using Finder. Then you should be able to do an "Add" import from the copies on the new hard drive with the "Do Not Import Suspected Duplicates" option unchecked.