Ever since hard disks were deemed critical to data processing, storage suppliers have devoted much effort to circumvent hardware failures. It started with basic disk mirroring and then evolved into the various RAID protection levels in attempts to reduce the cost of redundancy. As external disk subsystems became popular, vendors added redundancy to other components whose failure was considerably more catastrophic; fans, power supplies and disk controllers come to mind.
It’s now commonplace to regard storage products as offering “high-availability” (HA) simply because they have internally redundant hardware. This interpretation creates the expectation that you can always get to data on disks. But this is far from reality.
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