For anyone who really cares (probably no one), the system has now
transferred more than half the data over from the backup to the RAID system.
I have not yet experienced a single halt.  Performance seems good (I'll test
a  lot more once the data has transferred and I can take the backup array
offline).  With the RAID array more than 40% full, creating a file takes
almost no time.  Deleting a 23G file takes less than 0.9 seconds.