Can't speak for OP but I know some people use optical disks for cold storage backup, as they're reasonably cost effective there.

I can second that, I use M-Discs for long term backups. They're supposed to last 1000 years and you can get 100GB ones.

Thanks that’s very interesting. I’d never heard of these. They seem expensive but you get the millennium aspect (not something a spinning magnetic disk can claim).

Everything else set aside, the company lasted 7 years until bankrupcy, and that sort of casts a shadow on the millennium claim. There's at least nobody left as resposible, if it's not true.

I doubt they were ever actually responsible in terms of guaranteeing your data integrity?