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Data loss in parity RAID requires a second failure — or a third in RAID 6 — inside the rebuild window. The rebuild window is what makes the difference: a 37-hour rebuild is 0.42 percent of a year, so the conditional probability of a second failure is small but no longer negligible across eight disks. This model ignores unrecoverable read errors, which in practice dominate RAID 5 risk.
RAID failure model
P(first) = 1 - (1 - AFR)^disks; P(second during rebuild) = 1 - (1 - AFR x rebuild/8760)^(disks-1); RAID 5 loss = P1 x P2; RAID 6 = P1 x P2 x P3
Unrecoverable read errors. Reading tens of terabytes during rebuild at a URE rate of 10^-14 per bit makes encountering one quite likely, and that alone fails the rebuild.
No. RAID protects against disk failure, not deletion, corruption, ransomware or fire. It is an availability mechanism, not a recovery one.