LTO-4 tape (Score 1)

I don't recommend using hard drives for archival purposes, they're not rated for that. Have you considered LTO-4 tapes: the price per unit GB comparable and they are intended for archival storage. You could ship them to an offsite location or make two copies and send to two locations if you're paranoid. Setup is relatively cheap if you just buy a tape drive, somewhat more if you buy fancy software and a tape robot.