Looks like this site didn't post my response of yesterday. Anyway – thanks for all your repsonses! 🖖🏼
The HD is Mac OS journaled, but a First Aid run couldn't unmount it.
I moved the project to an SSD and it still had problems. Eventually I bounced in place the TWO audio files (which only had Slate Mixer on them), and deleted every other plugin and bus.
I managed to get a new third track down, and continue my work, starting the mix afresh.
My guess is the file has some corruption, and the 7200rpm HD is on its way to the big drive in the sky.