LR no longer recognizing photos/folders on HD
So, I have a 2TB external HD that I've been using for all my images for 2016. All have been imported into Lightroom. For 2017, I bought another (larger) external HD and I COPIED (not moved) all of the 2016 images/folders into the newer external HD as a backup. So, the original RAW files are still on the 2016 HD, unchanged, but now when I open LR to edit or view those photos, it no longer recognizes them as being there, forcing me to re-import them (thus losing all of my editing changes on thousands of photos). I can understand why LR would not recognize the images if I had moved them, but I didn't. I merely copied them to another drive, while leaving the original drive untouched. So why does LR no longer recognize the images on that drive? Using LR CC. I use one catalog for everything.