Thanks. I have spent the day reading about M-disc and optical formats. Lemmy is great for starting rabbit holes! I think I may swap a blu-ray burner into one of my machines and I am actually thinking dual layer blu ray is a decent trade-off: simmiliar chemistry to m-disc (unlike the clear superiority of m disc over DVDs -which I was literally burned on the other week with some yellowed DVDs), and yet still cheap, reasonable amount of disc shuffling, and in the future, nearly any blu ray drive will work with the 50gb discs (unlike 100gb disks). Still might grab a blu ray m disc burner if I see a good deal but they seem rarer than a hens tooth. Lol.

I have less than a TB of truly irreplacable stuff, so 20 discs isnt bad to suffle, and aiming for maybe 20 years of electrically safe cold storage* zero maintenance off-site seems pretty reasonable for a $50 drive and about that in discs.

*This is overkill anyway - already have zfs snapshots going to an external drive in another room and restic backups to b2. But the threat model is some combination of losing b2 (credit card fail or my own incompetence) plus housefire/robbery/electrical surge taking out everything. Also, if I get hit by a bus and my CC is cancelled, a box of transparrently readable discs are significantly safer bet for my survivors than my note about how to get the data off the NAS/backup drive. They could leave it 5+ years and come back to it no problem.