My data collection is small enough that I don't archive anything from my hard drive. (I do, of course, have redundant backups. [Hi Calvin!].)
But for those of you with large collections of offline archives, this might be a serious concern. I've just been reading File Not Found: The Record Industry's Digital Storage Crisis, an article on the Rolling Stone website.
Here's an excerpt:
Last year, the Beggars Banquet label unearthed the multitrack master recordings of the Cult's classic 1985 album, Love, for a planned deluxe edition. The LP was an early digital recording, and to the label's shock, one master was unplayable; the other contained only 80 percent of the album.
The article also mentions files made using obsolete formats or requiring plugins that don't run anymore.
Of course, even if you have all your data on your current drive, with multiple backups, that doesn't mean you'll actually be able to access that data. Maybe that app that created it won't run on your current OS or hardware, and maybe no other app can read the file.
Some formats are likely to last a very long time (PDF), but just because a format has had a long life doesn't mean it will continue to do so. For critical documents, printing might be a good solution. But that doesn't work for videos or music files. So if something is critical, you might need to migrate old data to new formats.