[em-bee]

The best way period would be to periodically backup to an external HD or even SD cards.

and where do you store those?

you really want multiple backups, and an encrypted cloud storage is one of them.

[tremon]

I have no trust in the longevity of data on SD cards. What's the expected lifetime of the data on these, and has it been studied/verified?

[suifbwish]

In a heavy fireproof box along with the birth certificate, and other important papers. If you want extra protection put that box in a larger safe. This really isn’t a difficult question to answer. If you are trusting a third party to store your things, when something happens to their service or they terminate your account randomly you have only yourself to blame.