Disc Rot: The Dangers of Dud Discs

Written on the 1st of April 2006 by Scott Jones (General Manager) IT Leaders

What is Disc Rot and Why Should I Care?

Disc rot is something that gives the lie to the widely-held belief that storage discs such CDRs (Compact Disc-Recordable) and CDRWs (Compact Disk-Rewritable) are a highly reliable medium for backing up data - that they will, in fact, last for 100 years. The horrible fact is that research shows some discs will start to fail in as little as two years, EVEN when stored in ideal conditions.

Why is this? It goes back to the way CDs are created, the quality of the CDs you use and the way you store them.  One of the original articles on the matter (as far as DVDs are concerned), was written by the Sydney Morning Herald in early 2003. Since then it has become widely accepted as an issue for all types of optical discs, although it was recognised as a rare problem in laser discs a great deal earlier.

To be fair, the term "rot" is incorrect.  This is a fallback to the days of laser disc (80s and 90s) when one specific manufacturer produced a batch of discs with a layer substrate that did rot.  Modern "rot" is actually layer oxidation.  Examine the picture above; see the damage on the left hand edge?

Writeable CD & DVD discs are constructed like a sandwich, with the data 'filling' enclosed between layers of plastic. The data is stored by an optical dye; as the disc is 'burned' by a computer's drive, the dye layer undergoes a chemical transformation that enables it to retain the data.

This means that one factor affecting the longevity of the disc is the stability of the dye, and some are better than others (see below).  But poor quality control in manufacturing can also cause layer separation in the the disc.  Any layer separation will cause air to be able to enter the space, and then the substrate layers can oxidise.  Because of the visible way the oxidation grows over time, it does indeed look like a disease of the disc, hence the continuation of the term "rot".

So dye quality or manufacturing conditions can mean increasing sections of data become unreadable, this can impact poorly stored and handled discs, as well as discs stored in ideal conditions.

 The following pointers can help you get the best performance out of your CDs or DVDs.

Be aware: you simply should not rely on discs alone to protect your data.

1. Use the right discs

Discs that use a newer dye called 'phtalocynanine' have been shown to be more stable, which gives greater storage life. The National Library of Australia, the ABC and the National Archives all choose phtalocynanine discs for this reason. The cost of the better quality discs is slightly higher, but we're talking cents, not dollars per disc, so it makes sense to make the switch. Imagine how foolish you'd feel to have saved a few dollars on discs only to lose hundreds or thousands of dollars' worth of data stored on cheaper discs that fail. The brand Verbatim have always made high quality discs in my opinion.

 Also, Re-writeable discs use a different kind of dye which will revert back to its original "blank" state when it is reheated. Do not use CDRWs or DVD-RW for long term storage as they are more prone to data decay.

2. Handle with care

Any handling of the disc can cause damage. Be sure to handle them by the edges, and avoid touching either flat surface. Contrary to popular belief, the top surface is MORE easily damaged than the bottom because it is only a thin layer of metal foil, which is attached to the dye layer.  Scratches and other damage will allow oxidation and cause disc rot.

Return your discs to their storage cases as soon as you've finished using them.

Write on discs only with water-based or alcohol-based soft-tip markers, NOT biros or solvent-based markers. Don't apply stickers to your discs but if you already have, don't try to pull them off as it can damage the disc further. Just don't do it again!

3. Proper storage is a must 

Store your discs in the readily-available storage cases or storage sleeves that have been designed for the purpose and store them upright, not flat. Be wary of disc cases which are poorly designed and make you bend the disc to remove it. This will cause layer separation in the disc.

Make sure they are stored away from bright light and are not exposed to high temperatures or humidity.

Recovering Data from a Bad Disc  

If you find a disc with disc rot, some or all of the data may be unaffected. You can try copying the data back to your hard drive in small chunks, and burning a new disc with the files which copied successfully. You can also try duplicating the disc.  There is no way you can damage your drive or your computer by trying to recover the data. Good luck!

Sensible Actions

Rather than getting to the stage of data recovery, what's much more worthwhile is to have a more potent back-up system than optical discs.

That is a whole new article in itself, so read here for a good backup plan for your business.

- First written Oct 2004 by Scott Jones for IT Leaders' clients.
- Updated by Scott Jones August 2006 for 'Your Life' Magazine.
- Updated by Scott Jones June 2007

Further business technology articles available from www.itleaders.com.au