CDs and DVDs are cheap and burning them is fast and easy. Make several copies, calculate the checksum, and store them in different places. Every few years see how badly they've degraded and after hitting some pre chosen threshold you can return all of them. If their data really is less than 1 GB they could even store 4-5 copies per DVD for additional redundancy. Even on a single disk I'd imagine the errors are unlikely to be in the same region on all 4 copies.