Pre-flood drives are shown to be incredibly reliable.
I have many (probably 20+ units) that I use for back up which are 15+ years old. (The last one that failed, like 1 out of 40, was so long ago I almost dont remember.)
Oh, ya. I run a couple of 120GB 7.2K WDs every day (2 hrs) & that's been going on since I bought them new at Fry's.
(The 320 Maxtors are even better. Those drives are stated to run like enterprise level.)
The approach is to just keep doing what you do as from a probability standpoint, it is not likely that the backup will fail at the same time that the source drive fails.
The probabilities are not additive, but multiplicative. Hence, if the probability of one drive failing is 0.01 then the probability of two of them failing simultaneously is more like 0.01 X 0.01 which is 1 in 10 thousand ! So replace the drive that fails
when it fails or shows evidence of failing.
The other thing is that drive reliability follows the famous "bathtub curve" (ie, high fail probability in early and late life with a long low failure probability mid-life).
(Checkout how many DOAs or early failures are reported on Newegg by users.)
You can buy inspected & tested pre-flood drives from DiscTech (
www.disctech.com). Because they are older vintage/technology, their performance will be less than current versions (but that's okay for backup). However, oddly enough, they will be more reliable than brand new having demonstrated run hours past the early failure point (of the "bathtub curve").