Just an update - I downgraded to 8.8 and re-ran the verification check - 14 errors. I had tried to cut out all disk activity on this volume from the point that I first saw the errors (the problematic volume isn't the system drive), but the downgrade reboot kicked off a process (now disabled) that updated some files, so I'm guessing (!) that those updated files must contain the new mismatching sectors.
Just to repeat the original questions:
1. Is there *really* no logging of where the verification errors occur? This looks like an unforgivable oversight.
2. How does the driver decide which of the mismatching sectors is "good" when fixing verification errors? Is one drive designated (invisibly) as the "master"?
Also, there were no media errors shown in any of the verification checks, but I can't see any way to access SMART data to see if either drive is reporting problems, so
3. Do the drivers report SMART errors in any way? (Does a lack of messages via the driver indicate that there have been no sectors re-allocated at the drive level?)