answer:It’s probably best to use several different media. CDs and DVDs are good because they’re immune to EMPs, though the jury is still out on how long the substrate will last before it degrades. Ordinary hard drives are pretty reliable these days, especially in a RAID configuration, though an EMP will take them all out at once. Having no moving parts, USB thumb drives are pretty immune to mechanical damage, but again could suffer during an EMP. So again, back up anything irreplaceable to different media. For me, my stories and manuscripts are irreplaceable. I keep backups on on a USB thumb drive. I also periodically copy them to several CDs (encrypting them in the process), and then keeping one in my car CD changer, and one at my mum’s house for added paranoia.