Changing LR from the original HD to the backup HD

You do not say which Lightroom you use, but it sounds like it must be Lightroom Classic. LrC stores its edits in the catalog, and stores the previews in the catalog folder. That folder is normally located on your internal drive, so it should be safe. To 'reconnect' to photos, all you need to do it make sure that the photos backup has the same name (Macintosh) or drive letter (Windows) as the original photo disk had.

If your catalog was on that toasted drive too, then I hope it was backed up too. In that case you need to start Lightroom from that backup by double clicking the catalog file on the photo backup disk. You only need to do that once.

Previews will be rebuilt when needed, but that shouldn't be the case.