What does "meantime between failures" mean?

Episode 1144 (1:12:56)

Richard from Grand Rapids, MI
Hard Drive

Richard wants to know what the "meantime between failures" means. Leo says that it's the time that a hard drive will live, or the average life span of a hard drive. But Leo says it's pretty much a made up number. They take 1,000 hard drives and run them until a handful die. Then they extrapolate how long the hard drives could last. It's mostly meaningless. Google and Rackspace have both released studies and 98% will last about 2 years, 2% die in the first year, and then they die at about 4% per year. That's just based on their experience.