Exactly as this thread says: https://discussions.apple.com/thread/8561149 I can't do a TM backup either to a drive connected with a cable or to a network drive. Both worked perfectly well until Mojave update. According to the thread above, Apple engineers don't seem to know what the problem is. I thought my network drive somehow broke, then connected a regular external SSD. Same problem. "Time Machine couldn't back up to [drive name]". Anybody else?