The TL;DR is that, IIUC, RAID depends on disks being 100% perfect with exactly zero read errors, ever, until they go *clonk* and die all at once. Today's many-

The TL;DR is that, IIUC, RAID depends on disks being 100% perfect with exactly zero read errors, ever, until they go *clonk* and die all at once. Today's many-TB hard drives use mind-boggling engineering to shrink data fluctuations down almost to the size of individual atoms (not quite there yet, but making progress (!)), while higher I/O speeds push the onboard processors' ECC mechanisms almost beyond breaking point. This basically means that the likelihood of HDDs returning uncaught bit errors has gone from a maybe-once-in-a-lifetime event (with very large magnetic flux sizes on disk in the 80s) to something that individual power users using disks with great-looking SMART output should generally expect to see every few months or years+. My rule of thumb is that any storage device over maybe 250GB-500GB in size needs to be redundant. If you can shove that much data into one place, the odds that something significant will be hiding somewhere in that data easily go beyond 100% once you go past that size range, IMO.