Almost three years ago, in "Is Your Data Disappearing?" we discussed the likely lifespan of burn-it-yourself CD-Rs--an important consideration when you're using CDs to archive data for long-term storage. At that time, even the cheapest CD blanks were thought to be good for at least 10 years after burning, with some premium rewriteable CR-ROMs possibly good for as long as a century.

Those estimates were based on accelerated aging tests performed on the various dyes used in CD-Rs or CD-RWs: The dye layer is what actually contains the data in a CD, and also is what gives the disk its characteristic color--blue, silver, green, etc. You can find abundant technical details in the original article, in "CD-R Media Longevity" or in Andy McFadden's ever-excellent "CD-Recordable FAQ", especially in the section called "How long do CD-Rs and CD-RWs last?"

But the conventional wisdom about CD-R longevity was called into question several months ago with a series of tests originally presented in the Dutch PC-Active magazine and widely recirculated on English-language Web sites, including the Register and Slashdot. Those tests suggest that many CDs may fail in as little as two years! Of course, accelerated aging tests--while normally quite reliable--are only a simulation of aging. There's no way that a carefully controlled laboratory test will precisely reproduce every random aspect of actual aging in real-life conditions. But two years seems very, very short, unless the storage conditions were poor. And some environmental conditions can be quite unexpectedly harsh on CDs. For example, way back in 2001, we reported on the existence of certain special fungi that actually can consume the dye, foil and substrate in some CD types! An article in the science journal Nature had said:

After visiting Belize in Central America, Victor Cardenes of Madrid's National Museum of Natural Sciences, found one of his CDs discoloured, transparent and unreadable.... The disk's aluminium and polycarbonate layers were riddled with fungus, Cardenes and his colleagues have discovered ... Burrowing in like worms from the side of the disk, "the fungus destroyed crucial information pits", says team-member Javier Garcia-Guinea. Pits in a CD's aluminium and polycarbonate sandwich store binary data, which is read by a laser ..." (Full story "Fungus eats CD")

But premature failures like that seemed like a fairly low-probability issue--how many people expose their CDs to rain forest fungi?--until the PC-Active article. That article sent many of your fellow readers (and me!) back into our libraries of old CDs to see how they're holding up.