No estimate on HDD's… "the jury is still out"
quote]According to David Handboek paper
"Hard disks are considered not durable; they have a relatively short life expectancy (due to heat, wear and tear)"
This is probably why the New Scientist article did not take them into consideration.
Irregardless of life expectancy, always remember that you need the capability to recover the data too - early tapes were a pain for this as the accuracy of the write head could vary larely between devices so a tape written on one, wouldn't necessarily read (a) on another or (b) on the same drive after 200 other tapes had been run through it.
Over my career, I've had a number of clients who have rooms of backup tapes, but no longer have _any_ devices that can still read them having upgraded years ago. The same can be said of PCs and HDDs SCSI disk interface anyone ? I rather liked the earlier suggestion of migrating 80% of the lifetime of the medium, but would extend it with "whenever your company is upgrading it's backup solution, or every 10 years whichever comes first". On the bright side, as capacities generally increase, you should gradually reclaim your storage room as you do this process, sooner or later you'll have your entire back catalouge stored on a single USB key 😉
I'm biased when it comes to long-term data retention, having actually performed that job in a previous life, and believe that tape is the only appropriate long-term solution given today's present technologies. LTO-4 tapes will get you 800GB uncompressed/1.6TB compressed with an average cost of >$30/tape, while LTO-5 will get you 1.5TB native, 3TB compressed at a cost of ~$80/tape. Lifespan of an LTO-4 tape is 30 years in ideal conditions, and if you're only making one large backup of a case at the end of its life-cycle and not things like incremental backups over time, then your LTO tapes should have no problem living a long time. Furthermore, LTO drives are backwards read compatible with previous generations of tapes, so an LTO-4 tape would be readable in an LTO-5 drive. That is, of course, something you would want to verify if doing a migration to the latest generation of drives.
Of course, the issue of maintenance and historical document format (as the referenced article points out) becomes a headache, if not a full blown migraine.
I know a CF examiner who did some work for a large electronics client a few years ago in a patent lawsuit. He was given ancient backup tapes with the original data on it. First, he had to find a drive and software to read the data, which he did thanks to ebay and a local ancient software seller. Then, he had to get the data out of MS Project 1.0 format and convert that to PDF files. So, yes, it can be a pain, but it is possible.
However, much of the issues with historical document formats is a non-issue when you consider that you have the software you're using to make images, you have the software you're using to make reports, and you have the software you're using for analysis. Archive that too. Hardware, thankfully, has a longer lifespan than software. I can still purchase hardware that I was using 10 years ago, quite cheaply, if I need any esoteric hardware needs.