Originally Posted by imagic
Flash memory, like what is found in thumb drives, is a far better medium for archiving than optical disc. With an optical disk, a manufacturing defect might take a few years to manifest. With flash memory, once the data is verified if you put it away it will simply be there.
If physical media persists in the future, it's hard to imagine that it would be anything other than non-volatile memory.
Untrue. Flash memory has a limited period of data retention - i.e. how long it will hold information when not under charge, in other words, if you wrote to it and then stuck it in a cupbroard. Whilst this period is quoted between five and ten years by manufacturers a lot depends on the quality of the memory. Genrally, the cheaper, the shorter.
Moreover, flash memory is very sensitive to environment, and can be affected by things like damp which causes corrosion and shorts.
Optical media are regarded as extremely stable over the long term and are the first choice for archival purposes, because they have high capacity, are cheap, resistant to environmental factors and have extremely long projected lifespans.
There is discussion to be had about writeable
optical media, because they are comprised of dyes which are inherently unstable, since they must have the property of changing state (otherwise, you couldn't write to them). Even so, the projected lifespans are very long. Most burned DVD's for example fail because many people buy very cheap and inferior products (which are often rebranded and sold under 'reliable' brand names) or they are kept incorrectly (for example, exposing a DVDR to a good dose of sunlight)
Pressed DVD (and CD) are regarded as the most stable of all because they are basically a layer of inert metal with pits in it (representing the 0'1 and 1's of the data) and are sealed inside impermeable plastic, these plastics having been around for a long time and their properties being well understood.
If a DVD fails then it is almost always down to a manufacture error (such as the use of incorrect materials) or bad storage (such as DVD cases that cause the user to have to bend the disc to get it out, which in the long term causes the bonded layers to separate, this delamination
finally causes exposure of the metal layer which oxidises and becomes unreadable)
Most manufacture errors show up relatively quickly, because the problems are chemical reactions which take days or months - not years. An example of this would be the DVD's from Anchor Bay that turned a deep bronze colour about six months after manufacture. The problem was the manufacturer used an incorrect laqcuer which chemically reacted with the metal data layer - which is made of aluminium - causing it to oxidise.
DVD's are far, far more resistant to "rot" than LaserDiscs. LD's were made using acrylic plastics which have excellent optical properties - this was necessary at the time because the lasers in players required it and error correction was much poorer. But acrylics are relatively soft and subject to moisture penetration. It was usually moisture penetrating to the data layer that caused "rot" - black spot sof oxidisation.
DVD's are made of polycarbonate plastics. Modern in-player lasers are quite happy with it;s slightly inferior optical quality and error correction is now much better. Polycarbonates are at least ten times more resistant to moisture than acrylics - DVD's have been immersed in water for many months without problems.
Properly manufactured and stored, a pressed DVD should have a lifespan that is considerably longer than it's owner's. In fact, the most likely fate of DVD's is that many decades from now there simply won;t be devices to read them. Although these are so trivially easy to make, it will certainly be possible to recover the data - even if you won;t actually be able to buy a domestic device to do it. Same way you just can;t buy a gramaphone to play old wax cylinders in the stores - but they can be made.