>> must keep for at least 3 decades > Most storage mediums won't have problems lasting that long if they're stored well. Particularly flash and SSD storage sho

>> must keep for at least 3 decades > Most storage mediums won't have problems lasting that long if they're stored well. Particularly flash and SSD storage shouldn't take issue: tape and HDD has a small chance of mechanical failure even if you don't use them. I think you're overstating the durability of these systems. I saved most every HDD I've used dating back to the early 90s. I wanted to get rid of the box of drives, so I got an adapter and tried to see what I could salvage from them before throwing them out. Ages varied from about 8 to 28 years. None of the ancient drives (>15 years) worked. I could usually hear them spin up, but I couldn't get anything off them. Several had the dreaded click of death present. I got some of the newer (<12 year old drives) to work. Even the newer 8 year old drives had really high failure rates. They were all working when I pull them out of service. My old USB flash drives have also had high failure rates for me. From my experience, expecting an HDD that isn't in use, to go for more than ~5 years totally idle is asking to lose the data on it. The data might still technically be there, but the mechanical+electrical system that makes up an HDD does not have high longevity. To the original question. I actually just researched this topic extensively to make purchases for my back up systems and my conclusion is: If I really want data to go 30+ years. I'm using archive grade Blu-ray discs (M-disc and LTH). I burn multiple of the data I want, and I store them in dark places at separate locations.