I went to the webpage. It says MKV makes ISO backups, so I remembered wrong. In the forum, somebody said they used DVDShrink to make an ISO. I downloaded DVDShrink and ran its media player, which was able to read the corrupted disk. After playing it in DVDShrink, VLC and Pot Player were able to play it, too. Now the first thing on the screen is the DRM warning, which didn’t come up before. Now the DVD is playable and backed up to PC. If you hadn’t been adamant about MKV backups, I’d still have a problem. So TYVM!