I have a simple rule - don't trust any single media or technology But seriously, the physical longevity is just one factor and it's often overlooked that backup is a *procedure* not just a bunch of data on a supposedly safe media. It doesn't matter if a DVD lasts for a 1000 years if I drop it face down and lose the data because of bad handling.

So, in my book, something I'd really call backup (and not just a spare copy) is something that exists at least on two different types of media in at least two different physical locations. DVD's are too small and too slow these days, had bad experience with RWs and I could make a landfill if I used Rs, BR is not really an option where I live ($$$), so that gives: 1TB NAS backup with the really important data periodically compressed/crypted/synched to Amazon S3.

PS. On flash wear: A while back I managed to run down a 512MB flash in a little over 6 months in 24/7 conditions (was used as a swap partition in a NSLU2, long story). I don't really see that as a potential problem in any real-life scenarios, though (you're not running your NIT swapping 24/7, are you ?).