*You* can "pretty much" guarantee it? Ok good. Great. If I have issues in the next half-century finding a Blu-Ray compatible drive that will work with a then-modern computer, I should come find *you* and you'll take care of it? I should only have 3-4 exabytes of data for you to recover by then.
Let me give you a few exercises so you can demonstrate the quality of your 'guarantee'.
- Find me a cassette drive so that I can pull some software for an Apple II from it.
- Find me a working 8" floppy drive that I can plug into my system.
- Find me a punch-card reader that will interface with a Windows 8 laptop.
- Now find me some software that will run on my computer (under either OS X 10.8 or Windows 7, your choice) that will allow me to read and recover the data I have stored in a Microsoft Works word processor file from 1993.
The difference between those and optical disks is that those older formats didn't go away until there was a clearly superior removable media standard to the older format which supplanted it. Punch cards were replaced by floppies. Cassettes were replaced by the CD. Floppies were replaced by recordable CDs.
And in each case, it took 15 - 20 years from the time the superior removable media standard came out for it to supplant the older standard. And compatible hardware remained (and still remains) available for the most popular of those standards for many many years later. I can still buy a 3 1/2" floppy drive today that'll work on a modern Mac. That's a 30+ year old tech, which has been obsolete for over a decade now.
Now, CDs, despite the advent of DVD and Blu-Ray, managed to keep a strong showing, because a conscious decision was made to use the SAME physical shape for the newer formats. So while few people still make CD-Rs, they effectively piggy back on newer formats like Blu-Ray and will be supported as long as those newer formats.
Add to that the fact that there is a HUGE volume of media being produced today on CD, DVD, and Blu-Ray, both commercially and by individuals and companies. There's no real sign that this is going to stop any time in the next decade at least. And there's NO sign right now of anyone coming out with a new removable media format which is superior to any of those without being backwards compatible.
(No - I don't count cloud storage as a workable replacement - although it has a lot of uses of it's own. It's a different sort of storage, but not really a competitor for removable media...)
If there were such a format on the market today, I'd be willing to say that Blu-ray's days are numbered - 15-20 years from today people might not be using them anymore. And maybe 30 years from now it might become hard to find a Blu-Ray drive.
But there isn't, and the odds are just as good that the next physical media format will decide to use the same disk format as CDs/DVDs/Blu-Rays for backwards compatibility. If that happens, there's no reason we shouldn't be able to read Blu-rays for many decades to come.
These are the issues with what most people think of as 'long-term' archives. Changes to undocumented formats, changes to hardware so profound that you've got to chain 3 or more adapters in order to allow you to plug it in, and that doesn't even guarantee it'll actually do the job at that point. And I'm not even talking about spans of time as long as the life-spans claimed by 'archival quality' optical media today. (Most notably because we weren't using electronic, or even electromechanical, computers yet that long ago.)
The fact of the matter is you *can't* guarantee it. You can't even guarantee that the 'archival quality' disc will last more than a decade. In practice, many 'archival quality' CD-Rs, with manufacturer estimated life spans of 50+ years, failed inside of 10 years despite being stored according to the manufacturer's recommendations. Burnable DVDs are a bit better, but still have the same root issue. Why? Because those life spans are estimated (based on standard tests, yes, but those test give numbers that cannot be verified, because even *pressed* CDs haven't been around long verify a life-span that long). Even 'archival quality' discs commonly have an *unburned* shelf-life of 5-10 years, even the *best* 'archival quality' discs have dyes which will degrade in less than a month of exposure to sunlight.
No, you can't guarantee that *any* individual instance of media will last as long as its manufacturer claims, but no optical media labeled 'archival quality' by it's manufacturer has lived up to those claims in practice yet. (As you said, CD-Rs, one of the earliest wide-spread writable optical formats, are less than 30 years old, and the manufacturers claim terms longer than that for their life spans, though quite a large number of those discs have since failed.) Pressed discs, I'd give decent odds on that front, but pressed discs aren't used or archives. They're used for mass reproduction of the same data on thousands or millions of blanks.
Even with proper storage, the
National Archives site recommends that you test your optical archive media "at least every two years to assure your records are still readable", presumably so you can catch the 'bit-rot' in the dyes before it exceeds the capabilities of the format's error correction measures. And this is in their FAQ on how to deal with storing *Temporary Records*.
Professional archival groups are busy looking for *actual* long-term storage solutions, because none of what we have now actually fit the bill.
You think I'm unaware of these things? I've been dealing with archival issues for 30 years - I know all of the issues with them. It doesn't change the fact that right now (and for at least the last decade), there is simply no more effective archival media out there for your average small company than optical media. DVDs are still the most cost effective, but Blu-rays are quickly getting to the point where they'll eclipse them. I've already had to deal with clients who mistakenly believed that external hard drives were acceptable for archives - they simply fail WAY too easily, and often in only 3-4 years. And don't get me started on Flash Drives...
(And you think the long term support issues for Blu-rays is a problem? That's a *trivial* problem compared to long term support issues for hardware interface formats. SCSI and Wide-SCSI are already nearly impossible to find, and even IDE/ATA is starting to become pretty difficult. Hardware interface formats always thresh faster than removable media because removable media drives can always be made foe new hardware interfaces. That said, I'll predict USB lasts at least 20-30 years as well for much the same reason CDs/DVDs/Blu-rays will be around that long - but I'm less optimistic right now about the long term for Firewire, SATA, or Thunderbolt...)
If you're a big company with a lot of funding, there are other solutions, but for smaller companies, stick with optical disks.
FWIW, the vast majority of archive quality disks seem to hold their data pretty well. I've recently recovered data off of 20 year old CD-Rs with no issues whatsoever. Yes, the National Archives suggests testing regularly, and I agree - no matter WHAT storage medium you use. I also strongly recommend triple or better redundancy on archiving anything remotely important - and storing them in different locations. Because no media is 100% foolproof, and no storage is 100% disaster proof.
