Why CD’s go bad
Sam called this morninn wondering if he might be able to recover data that seems unreadable on several cd’s. I didn’t have a good answer for him, but here is what I found out. Discs are created in an interesting manner (note that this is a basic description). Plastic is laid down, then metal reflective foil, then dyes are poured onto the foil. Another plastic is laid on top, then the disc is spun at high speed to spread out the dye, hopefully evenly. It even sounds like an unstable method!
Dye imperfections. Bad foils and dyes, as well as bad dye spread are the most common issue that causes bad media. If the dye is uneven or does not reach perfectly to the edge of the disc, it is often bad.
User error. A common “error” with “bad media” is actually user error. Even I’m guilty of this. Do not try to use your computer extensively while burning, especially at 4x or faster speeds. Also test your discs before dumping the source. Run several tests, as the “verification” features found in programs like Nero Burning ROM have been known to not properly catch errors.
Problems past the 4GB mark. Balancing is also a side effect caused by faulty plastic, foils or dye spread. Round objects tend to be most unstable at the outer edges. While DVD media allows for some degree of error (data is written in a “wobble groove”), exaggerated wobble will caused the laser to spew data in areas not meant for writing. It thus disappears, and the data comes up as missing on the disc, resulting in freezing, blockiness and other odd visual errors, which are caused from the decoder attempting to compensate for material that is missing.
Fake media. Fake media is often bad. If you ever acquire good media like TDK, Taiyo Yuden or Maxell, and the results are bad, check to see if the media is legitimate. Many of these top-tier media companies only have branded discs, not plain white-top or silver-top ones. These discs often are cheaply-made unbranded media with a faked write strategy and media ID code. Fake media is often sold on eBay and by unauthorized online merchants. If you want to have guaranteed legitimate media, only buy that media from authorized media resellers. Visit the disc manufacturer Web site to get a listing of authorized distributors.
Gradual data loss (dye media). Also known as “disc fade” and “laser rot”, this is actually not very possible due to the mechanics of dye-based DVD recordable media (DVD-R, DVD+R). See the longevity page for more on this topic. Gradual data loss (phase change media). Unlike the dyes found in write-once media, phase change crystals are subject to deterioration, sometimes at a disturbingly fast rate. Whether the discs are used or not, phase change crystal can begin to break down in as little a six months! Phase change media includes DVD+RW, DVD-RW and DVD-RAM. See the longevity page for more on this topic.
Dye melting from excess burn speed. Some discs simply cave in under high speed burns. In the past, for example, both OPTODISC 4x DVD-R and RITEKG04 4x DVD-R media were reported as doing this when burned at 4x or 8x speeds. The dye simply cannot handle the write speed. Poor production is to blame. Hacked firmwares that allow overspeed burning has also at fault. The dye appears discolored or otherwise unusual.
Not a media error. This is another very frequent “problem” with discs. The simple fact is not every player or DVD-ROM will read a burned DVD media. Some players were not made to play anything other than official to-spec pressed metal discs. Non-media errors may also include players that have a weak laser or a dirty laser. A non-playing disc is not a sign of a “bad” disc, but rather just means that particular player/ROM cannot play it. Run real tests to see if the disc is truly a coaster. Also be aware that some formats are more compatible than others (with DVD-R being most compatible of them al for DVD-Video content). This is often call a reflectivity error. This is not necessarily the media’s fault.
There are tests that he can run that may help him in the future and possibly tell him more about the unreadable disks he has now. Check out http://www.digitalfaq.com/media/burnquality.htm