Hmm seems I spoke too soon. Just got another couple of "disk ejected" notifications, even tho' the hard drive is plugged into a powered hub. Still, it worked fine for 48 hours which is better than ejecting (and crashing) every 30 minutes or more frequently. The latest occurred after rebooting the Mac with the hub plugged in, I'm going to try only plugging in the hub after booting to see if it is more stable. Definitely seems to be a power issue.

It seems the problem is that Big Sur and Monterey are using more power than Catalina, and not delivering enough power to the hub to keep the hard drive alive. In my case Monterey crashed every time Time Machine finished which was driving me mad. The solution is to use a powered hub, no disk ejection since I installed one (or you could throw away the perfectly good hub and hard drive and buy a powered hard drive, sending Apple the bill).