Magnetic disk are not a permanent storage, AFAIK not even for medium, they break, when they mustn't. Optical such as blueray kept in a really safe place; hidden from light, humidity and temperature variations can last much longer.
Hard Disk failure may always occour, i had a raid 1 at home.. both disk failed at the same time...
You may even do something good such as compressing your data and adding some disks for additional safety with some kind of error correcting code spreaded in them.