You should be able to improve over naive RAID by pairing a relatively-high-probability-of-failure drive with a low prob one. i.e. what you *shouldn't* do is the common practice of putting two new drives in a mirror, since they are both in the infant mortality part of the failure curve. What this data suggests is that you'll get a smaller chance of losing data (via simultaneous failure) if you pair a new drive with an older "proven" one (but not one so old that it is nearing end of life).