Lost music. It was an underground artist who quit in the mid 2010s before archive culture was really a thing. I’ve had people dm me looking for it before. I keep it offline only out of respect for the author who wants to stay gone because the reason he quit a decade ago was harassment.
at the same level of importance, every time I’ve gotten a new computer or operating system since 2013 or so, I’ve backed up every single user folded (pictures, videos, documents, etc) onto an external hard drive. I have a lot of good memories there that I really do not want to lose.