Prob of no failure in 1 drive is 95%. Prob of no failure in 4 drives is .95^4 = .816. Prob of failure in any of 4 drives is 1-.816 = 18.6% Disclaimer: I got

Prob of no failure in 1 drive is 95%. Prob of no failure in 4 drives is .95^4 = .816. Prob of failure in any of 4 drives is 1-.816 = 18.6% Disclaimer: I got a D in Probability & Statistics