What is the system's mean time to failure? I have the following homework problem:

A 10 TB disk drive has an MTTF of 6,000,000 hours.  How much data can we store in a system comprised of these disks, if we want the system MTTF to be at least 1.2M hours?
If we are allowed to make it redundant (i.e., two of them operating in a parallel mode), and if the MTTR is 120 hours, what is the system MTTF?

I think it is
$\qquad \operatorname{MTTF}(\text{System}) = \frac{1}{\sum_i \frac{1}{\operatorname{MTTF}_i}}$.
For part 1, its simple. You need to find the $x$ (as defined below) for which MTTF(system) = 1.20M.

So $1.20 * 10^6 = \frac{1}{[(\frac{1}{6*10^6})*x]} \implies x = 5$.

We can have 5 disks (50 TB) if we want an MTTF(System) = 1.2M.

Part 2 is homework for you.