Not the cleanest solution, but mdadm info showed /dev/md0 was mirrored in a 2.4G partition on both hard drives. Didn't want to risk hosing it altogether by making changes to just one over a USB SATA adapter, or trying to rebuild the array over two of 'em, so went the annoying but safer route of just whacking reset pin on the back and reinstalling the firmware. Handled it quite well, only re-mke3fsing /dev/md0 and requiring a handful of settings + my 3rd-party ipkg tree be reinstalled. The downloader remembered its task list and the data partitions were untouched.
Not bad for a $100 NAS. Never found a clear cause (no lsof available, etc, etc) but fairly confident the cause was just frequent city power failures.
Thanks for the advice, Sean