Well, by that measure it doesn't even matter if a drive fails with 10% or 5% probability beyond the replacement costs. Because a test with 50% sensitivity effec

Well, by that measure it doesn't even matter if a drive fails with 10% or 5% probability beyond the replacement costs. Because a test with 50% sensitivity effectively halves the failure rate you have to use. Since it's all probabilities, triple redundancy does not guarantee complete absence of data loss. On the other side of the spectrum, single replication might offer a better cost/safety ratio for some applications. A failure that is known in advance is equal to no failure in these term and even when discrete, it will make the difference once in a while.