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True with two independent servers at 90% each, that’s 0.1^2 = 1% chance both fail— so redundancy can add a lot of reliability.

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Only if they are truly independent of each other.

You and AWS are using similar chips similar hard disks even with similar failure rates.

If you both use same hardware from say batch both can defects and fail at similar times.or you use the same file systems, that say corrupts both your backups.

90% is not a magic number , you need to know AWS supply chains and practices thoroughly and keep yours different enough not to have same risks as AWS does for your system to have independent probability of failures.

True. One would want to continually decorrelate services or model the dependencies. Redundancy will help even with some dependency, but you raise an important point.

You assume failures are uncorrelated. Which, depending on what you think you are protecting yourself from, might or might not be true.

(Consider a buggy software release which incorrectly deletes a backup. Depending on the bug it’s very possible it will delete in both places.)

If one buggy software release can delete both copies, then you don't have actual redundancy from the point of view of that issue.

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