Want a real archival backup that your great-great-grandchildren will be able to read?

Most media (CD-Rs, tape, disk, I think also flash) decay after 10 to 20 years. Having lots of redundant copies helps a little, but only a little.

If you want things to last a LONG time (say, 100+ years), I think the best options today are:

1 – M-DISC BluRay discs. They are designed to last 100 years. Of course this hasn’t been tested – the number is based on projections and knowledge of the decay mechanisms. From what I’ve read the chance of them being readable after 200 or 300 years is pretty good, if they’re stored in a dark cool place (say 5 to 10 C). Probably purging the oxygen from the container (flush it out with nitrogen) is a good idea too.

And, of course, multiple redundant copies.

2 – Multiple external HDDs, but keep them running the whole time (bearings often seize up after a few years if they’re not run). Actively migrate the data to new hardware every 10 years or so. (This requires money and effort of course.)

My understanding is that the ZFS (Zettabyte File System) is the way you’d want to store data across multiple HDDs – you can adjust the redundancy level as you like and ZFS will use all available space to create more redundancy if you want (as you originally where thinking).

3 – If you only have a little bit of data to store (< 1 MByte) – punched paper tape or punched cards. Store them in sealed containers purged of oxygen, in a dark cool place. If you’re really serious, use punched cards made of out of solid gold (gold is inert), and put the sealed container in a Nazi submarine and sink it to the bottom of an ocean.

Are you concerned that 100+ years from now nobody will have the hardware to read the media? Don’t be. If civilization doesn’t fall, it won’t be a problem (if it does fall, yes a problem).

I don’t think there’s any storage media used 100 years ago that we can’t today build a new reader for – easily and cheaply. And of course there will always be historians and museums who keep readers functional for common media.

Re uncommon media, a few years ago there was a project to recover data from some 50 year old tapes with data from NASA spacecraft. There were no working tape drives that could read it, so they built a new one from scratch. Anything we can build today at all, the technology of the future (assuming civilization doesn’t collapse) will find easy to duplicate.