Which cold backup media is out there, and what is the best?

[Rusty Proto]

I'm looking for something I can connect in via USB every so often, about monthly or so, to keep an indexed copy of my stuff under Linux with multiple file versions. While something like tape or M-Disk sounds best, as it will be intended to be hidden away somewhere, I am seeing many people using hard drives for the purpose, which I will have two 4TB SATA disks that I use as a hot backup and scratch array, then transferring to cold storage periodically. Most of my data fits within about 512GB or so atm, and I will be primarily backing up $HOME. What would be the best simple option for cold storage?

[8tg]

How long do you intend to hold onto this or expect it to last?

mdisc is probably the best for super long term but even conventional hard drives can go decades without issue, I’ve got drives from the 80’s that still work and theoretically so long as they aren’t used heavily they can last idle for a century or more.

 

I would just use a pair of external high quality data center drives good for extended writes, one for redundancy.

[Rusty Proto]

I'm actually intending to maintain archives for 20-50 years or longer if possible, pretty much a lifetime. Although, what concerns me about hard disks is mostly them seizing up mechanically if they are kept somewhere suboptimally (such as most out of the way places I have) as well as to be able to resist an occasional disaster.  Clouds are good for just keeping a little bit of data, and have a habit of losing data for me. I really need some sort of local solution that lets me keep a copy behind the couch, on a shelf in storage, and maybe one in the attic of my family's house, and that I know I'll be able to read as technology advances.