It means when you have a thousand disks, you lose one every month (or a dozen every month if this paper is correct: "We find that in the field, annual disk repl

It means when you have a thousand disks, you lose one every month (or a dozen every month if this paper is correct: "We find that in the field, annual disk replacement rates exceed 1%, with 2-4% common and up to 12% observed on some systems").