Write once DVD (DVD-R; DVD+R) media is pretty stable - good for at least 10 years if kept away from heat & sunlight. Archival versions claim stability of 100+ years if properly maintained. It is cheap to buy, making multiple (or periodic) copies an affordable option, and doesn't have the mechanical failure potential you find with a hard drive. The enemies of optical storage are typically heat and UV. Also, you should avoid add on paper lables, they have the potential to jam some readers as well as the ability to pull/tear the backside reflective layer. Inkjet printable discs are less prone to damage from felt tip labeling (from grit in the nib) than bare discs as well as giving a more legible result.
Retrieval software availability for archived data is a real concern. If you maintain actual document images in a universally readable format (.jpg, .png, .tif, image based .pdf, etc.), as well as any proprietary file formats being generated, you don't have to worry so much about being able to get at the data at a later time when the current software or operating system may not be available.
Nick
