My personal external backup is two external drives in RAID1 (RAID0 wtfff?). One already failed, of course the Seagate one. It failed silently, too - a few sectors just do not respond to read commands and this was discovered when in-place encrypting the array. (I normally would avoid Seagate consumer drives if it wasn't for brand diversity. Now I have two WD drives purchased years apart.)
It's a home backup so not exactly relevant to most of what you said - just wanted to underscore the point about storage media sucking. Ideally I'd periodically scrub each drives independently (can probably be done by forcing a degraded array mode, but careful not to mess up the metadata!) against checksums made by backup software. This particular failure mode could also be caught by dd'ing to /dev/null.
ZFS really shines here with its built-in "zpool scrub" command and checksumming.
Even though I am preaching "application consistent backups" in my original comment (because that's what's important for businesses), my home backup setup is quite simple and isn't even crash consistent :-) I do: Pull via rsync to backup box & ZFS snapshot, then rsync to Hetzner storage box (ZFS snapshotted there, weekly)
My ZFS pool consists of multiple mirrored vdevs, and I scrub the entire pool once a month. I've uncovered drive failures, and storage controller failures this way. At work, we also use ZFS and we've uncovered even failures of entire product lines of hard drives.
It's a home backup so not exactly relevant to most of what you said - just wanted to underscore the point about storage media sucking. Ideally I'd periodically scrub each drives independently (can probably be done by forcing a degraded array mode, but careful not to mess up the metadata!) against checksums made by backup software. This particular failure mode could also be caught by dd'ing to /dev/null.