Ah! I had so many of my beta users ask this precise question that I did some research and wrote up this article: https://photostructure.com/faq/how-do-i-safely-store-files/

TL:DR; Have several copies on different devices. You might want to invest in a NAS. Extra credit if a copy or two is not in the same place. Recent NAS and Linux distributions support filesystems that can guard against bitrot, which may not be common, but a layered data security approach is best.