Storm said, "I'm past 55 hours total run time. 286 million primes found. Current max value is above 6 billion."
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Terrific! Your program's collection rate now exceeds mine, assuming that you have found all the prime numbers. I have collected and stored only 70 million of them and I imagine the largest one is still much less than a billion in magnitude.

However, I'm only using one file to store them all. You said you are using several thousand files? How do you intend to track all of them? Lose a few and you are toast.