It's not unreasonable to believe that if you pick two identical products off the same shelf at the same time (as one would logically do when purchasing 2 of a single item), that the two products were manufactured at similar times and in similar conditions.
Your model isn't exactly bad, but there is an assumption being made that you haven't accounted for. Which to be fair, is frequently not stated. The assumption is that the drives defects are independent of one another. This is a poor assumption when manufactured back to back.
Your model isn't exactly bad, but there is an assumption being made that you haven't accounted for. Which to be fair, is frequently not stated. The assumption is that the drives defects are independent of one another. This is a poor assumption when manufactured back to back.