With that volume, hard discs do seem to be the most practical option. I think I'd want to two duplicate hard discs for each year.

The empeg community has prover that hard discs can store data for a reasonably ling period, how many of us have 10+ year old discs that still work and the data written 10 years ago is still readable...

Also I'm guessing that the nature of RAW files means they are good candidates for recovery even when hit by a couple of unreadable blocks.