I think someone else mentioned in this very comments section that hard drives "rot" while spun down — not the platters, but the grease in their spindle motor bearings or something gets un-good, so that when you go to use them again, they die the first time you plug them in. So you don't want to use offlined HDDs for archival storage.

(Offlined SSDs would probably be fine, if those ever became competitively affordable per GB. And https://en.wikipedia.org/wiki/Disk_pack s would work, too, given that they're just the [stable] platters, not the [unstable] mechanism; they would work, if anyone still made these, and if you could still get [or repair] a mechanism to feed them into, come restore time. For archival purposes, these were basically outmoded by LTO tape, as for those, "the mechanism" is at least standardized and you can likely find a working one to read your old tape decades later.)

Even LTO tape is kind of scary to "leave on a shelf" for decades, though, if that shelf isn't itself in some kind of lead-lined bunker, given that stray EM can gradually demagnetize it. If you're keeping your tapes in an attic — or in a basement in an area with radon — then you'd better have encoded the files on there as a parity set!

I think, right now, the long-term archival media of choice is optical, e.g. https://www.verbatim.com/subcat/optical-media/m-disc/. All you need to really guarantee that that'll survive 50 years, is a cool, dry warehouse that won't ever get flooded or burnt down or bombed [or go out of business!] — something like https://www.deepstore.com/.

But if you're dealing with personal data rather than giant gobs of commercial data, and you really want your photo album to survive the next 50 years, then honestly the only cost-efficient archival strategy right now is to keep it onlined, e.g. on a NAS running in RAID5. That way, as disks in the system inevitably begin to die or suffer readback checksum failures, monitoring in the system can alert you of that, and you can reactively replace the "rotting" parts of the physical substrate, while the digital data itself remains intact. (Companies with LTO tape libraries do the same by having a couple redundant copies of each tape; having their system periodically online tapes to checksum them; and if any fail, they erase and overwrite the bad-checksum tape from a good-checksum copy of the same data — as the tape itself hasn't gone bad, just the data on it has.)

Paying an object-storage or backup service provider, is just paying someone else to do that same active bitrot-preventative maintenance for you, that you'd otherwise be doing yourself. (And they have the scale to take advantage of shifting canonical-fallback storage to being optical-disk-in-a-cave-somewhere format — which reduces their long-term "coldline" storage costs.)

Instead, you're just left with the need to do the much rarer "active maintenance" of moving between object-storage providers as they "bit-rot" — i.e. go out of business. As there are programs that auto-sync between cloud storage providers, this is IMHO a lot less work. Especially if you're redundantly archiving to multiple services to begin with; then there's no rush to get things copied over when any one service announces its shutdown.