It appears as though every time rutabaga is powered down the RAID array loses its marbles.
This time the new drive got dropped from the array. In fact, the drive didn't even show up in 3 reboots. I just powered up the machine and now it *does* show up. So I went through the same process as previously described, and I'm rebuilding the array right now. So far all appears OK.
UPDATE: array is now rebuilt (15:20) and all appears to be fine.