I've been working with computers since the early seventies. None of the storage devices that were in use for the first 10 of those years can be read today unless one digs up a working device from some museum. Even the early PC HDDs are becoming a challenge, since the bus interface have been changing, and now we are talking about units that are sometimes less than 20 years old. I'm sure there are many people out there with old backups on HDD units that they will have difficulty reading because of this. 20 more years? Forget about it.