A lot depends on who wants the data/music/whatever and for how long. If it's an archive of material that might be important for decades or centuries, you store it in controlled conditions, you possibly keep copies in different physical formats, and you re-copy it to new media before the old ones wear out. If it's personal stuff you make your own decision how much trouble is worth taking.

That said, I put some of my photographs into a "Camera Roll" folder on my notebook computer for a slide show at my birthday party last year, and some of them that I remember appearing in the slide show were missing when I went looking for them yesterday. The same ones are also missing from the folder (and subfolders) which, as far as I knew, contained all my photos.