I suggest what I heard sometime back was the industry standard of three backups in two different locations for anything you *can't lose.* Two locations, because what if something happens to one location--fire, flood, theft? Three copies, because even with only three, you may have issues with "version control." You will have to meticulously keep track of them, or you will go nucking futz trying to remember which version is current and which one you made which modifications to. Most professional backer-uppers I am aware of have calendars and procedures for this. By the way, the major operating systems now have built-in backup features.

If you are concerned with the media itself, I would suggest also using another media type, like CDs. CDs are dirt cheap, anymore--less than a dime in bulk. And as we know from our music, CDs last a long time. I have many that are over two decades old. Remember NOT to mark them with a Sharpie. The ink can bleed through and damage your data. There are special pens specifically for this purpose. USB external hard drives are also very useful, and not too expensive. But I am answering more questions than you asked.

I will further investigate your question about media longevity.

Saul