First of all, permanent is a hard challenge, and CDs (as you mentioned) generally was good somewhere between 1 and 10 years, not more. I know the danish nationa

First of all, permanent is a hard challenge, and CDs (as you mentioned) generally was good somewhere between 1 and 10 years, not more. I know the danish national library spends quite some time transferring archives from CD to a newer CD. I think backup tapes are still the best bet (as long as they are LTO-based). Built for longetivity (some/most have stated lifespans of 30 years on a shelf, YMMV) and an open format (i.e. three completely separate business building drives that can read each others tapes). The LTO tapedrives also do in-flight test of the writes (at full speed), so that an failed write will never be unnoticed. Actually they have a write head, then a read head then another write head and another read-head. When you write to the tape, writehead1 writes it and readhead1 checks it. If it fails, it retries with writehead2/readhead2 .. If that fails too, the backup is aborted and the tape is marked (in tapemedia-internal memory) as bad. The bummer for a private person is that even though the tapes themselves are relatively cheap (around 200USD for a 6 TB tape, that can take up to 15TB depending on compression rate of your content) the drives are expensive as heck (due to a lot of smarts), think 5k USD range. Another benefit is that if a drive goes bad, it can be exchanged without the datamedia being exchanged as well (the same is true for CD/DVDs but not for harddrives or SSDs). But as always, if you don't have three copies (on different media/sites/...) of your data, you actually don't have any copies at all.