What's more relevant is that the odds of either drive failing are identical. The likelihood that the first drive will fail doesn't change because the second one failed -- which is the classic statistical blunder. ("I got 42 heads in a row, the next one's BOUND to be tails!")

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.