Not along my normal lines of fixing, but I get into a little of everything. In my quest to digitize our media library I've stumbled across a few scary things with CDs and DVDs that may change the way you think about these as a 20-50 year "archival" media.
If any of you know my work history of making solar panels, I've always been the "bricks and mortar" type. Use thick sections of silicon instead of thin films because there's not much there, that's what I always say. Apparently that transfers across technologies, as the thin films used to produce CDs and DVDs are subject to environmental factors that degrade them faster than you would believe, sometimes less than 10 years.
In digitizing probably over 300 CDs and 200 DVDs, I've found maybe 2% so far that have become unreadable by a modern computer/data grade drive. There seems to be two primary failure mechanisms, either the lacquer coating to protect the data surface was damaged (and I'm not talking scratches) or the data layer itself has corroded.
On CDs, the data is pressed into a polycarbonate disc blank and then flashed with aluminum in a vacuum chamber before being spin-coated with a lacquer and made ready for silk-screening of the design. The final disc is read through the bottom polycarbonate blank. If that lacquer top coat does not seal the edges well or the aluminum is not masked or removed from the edge before lacquering, the aluminum is exposed and will oxidize and react with moisture causing the reflective layer in the CD to wrinkle. Picture aluminum foil after you've crumpled it...
DVDs are a little different, and particularly 2-sided ones. 2-sided discs are pressed or laser cut both sides and a final lacquer applied. In this case, the laser reads the disc through the lacquer coat, so if the coating is not applied properly, the disc not cleaned properly, or a sub-standard or contaminated lacquer is used, over time the layer can be permeable to moisture/oxygen and corroded the data layer, or the lacquer itself absorbs moisture and begins to take on an "orange-peel" texture. Obviously not good for reading or focusing lasers to spot size ~0.8 microns.
So what to do?? Some brilliant people created an open source program for Linux called "ddrescue". Unlike "dd", which is a command line tool to write raw data to and from discs/files/whatever in sequential order, "ddrescue" works around a read error from the source and it skips ahead and keeps going, and flags that area for another attempt at some point. It is exclusive of "dd", and uses it's own algorithm to attempt to retrieve as much data as possible. It can even reconstruct a file/drive from two partials, say two drives of mirrored RAID setup that failed in different areas. It has its limitations, but it's pretty darn good.
How to do it?? Understand what your doing, but essentially, within Linux you treat the CD/DVD as a data disc and recover it to the system hard drive sector by sector. In my case, I had a side of DVD that was unreadable and truncated a 43 minutes program to 23 minutes and the following on from 43 minutes to 5 minutes. After letting ddrescue chew on the disc for about an hour, it had recovered all but 69kB and 173kB from two source files, or less than a second of video. The transcoding reader to digitize the files easily skipped over the error sections and there was hardly a minor blip.
NOTE: All of my work for digitizing is for private/personal use only. Whatever you do, understand copyright laws before you do your own work.
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