[Dylan16807]

> - The chance of any given bit (of data on a disk) going bad, in a given time interval, is relatively constant.

We can't just assume this, though. Especially for data that's being checked regularly, because the drive can see how many pre-ECC bits have gone bad and rewrite iffy sectors.

And even if it's true, it's normal to go years without errors. If you have a 15% chance of getting an error each year, and your degraded array has 4 drives without redundancy, then the chance of getting an error in 4 hours is .02% and your chance of getting an error in 48 hours is .3%. Having that risk show up once every few years is pretty negligible compared to everything else that could go wrong with your array.

On top of that, if you don't have a hot spare and it takes 5 days to get a new drive, the difference in resilvering speeds is now 5.2 days versus 7 days and that barely matters.

The main situation where you have to worry about extra drives failing is when you think the resilvering process will greatly accelerate drive deaths. And that depends very much on the specific numbers you have in mind.