Basically - always back up to 2 separate external hard drives and if one dies just replace it with a new one because it's usually way cheaper than fixing it. But keep 2 copies of everything.
No one knows the corrosive potentials of digital files and storage mediums. The trick is to keep duplicates of files and adapt with the times.
It's likely in decades to come that file sizes and storage mediums will shrink drastically and all our current films, music and eBooks will become and obsolete medium. However they will still need to be preserved during our life times.