NOTE: The boot drive above is HD2. HD0 and 1 are both empty but partitioned and formatted NTFS (planning to switch to them exclusively when Win 7 launches).
Changing the AHCI Controller driver from the default (Microsoft 18.104.22.1683) driver to the Intel driver (22.214.171.1243) seems to have fixed this problem. I obtained this driver by downloading and installing the latest Intel Matrix Storage Manager for this chipset/MB.
I'll run it for a couple of days and update the thread with results.