I have the latest version of LR Classic on my Mac Pro 2013, also latest OS. The catalogue and associated files are all in the same folder on this boot drive, my photos are on an external drive connected by USB 3.0. My problem is this: I have made Standard sized previews of all of my 60,000 photos and several times in the last few months they seem to have disappeared, and I have to ask LR to re make them, which takes many hours. I have not moved or renamed the LR catalogue. Any thoughts about what might be going on?