Lots of people who didn't fail statistics assume that events are independant.
A bit like, the chance of my machine catching fire are 1 in a 1000, my machine did catch fire and I lost both drives - the odds on that happening must be 1 in a million!
Every single drive in the datacenter typically has the same likelihood of failure (since they're usually the same make and model, and similar production runs), so the odds that a drive somewhere in a large data center is failing RIGHT NOW are in reality rather high.
The Big Mac ended up being a good example of this. The likelihood of having a memory error on a normal PC is low, because the likelihood of a single DIMM having a memory error is low.
Punch that up to the 22,000 or thereabouts DIMMs that populated the Big Mac cluster, and now you're looking at a very HIGH likelihood that a large-scale computation using the cluster will experience memory errors, and therefore produce invalid results -- which is why UVa ended up replacing the entire cluster with ECC-equipped machines in short order. Until they did that, researchers had to run simulations multiple times and compare results to make sure that their simulations weren't contaminated by memory errors.
No thats the point, hardware failures on the same machine/rack/psu/site are not independant. A power supply spike that kills drive 0 in a RAID will probably kill the mirror drive 1 as well - that's why RAID isn't a bckup stratergy
I was explaining a common misconception about statistics, and using hard drives and DIMMs as examples.