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"Permanent" Data Storage
Over the last few years CDs, and more recently DVDs, have become a popular way to back up computer information. Most people think of these as a safe and permanent method of archiving. They are much more reliable than tape systems have been. They aren’t susceptible to magnetic fields and never get tangled around the recording head. The reality is, however, that they do have a practical limit to their lives. That lifetime can be severely compromised by improper handling and storage.
Several layers of materials are compacted together when the disks are manufactured. DVD’s have multiple recording surfaces, which allows them to hold much more than CD’s. As long as the layers remain stably attached the disk should read accurately. The Read-Only versions of the disks get physical "bumps" molded into them internally when they are recorded to. These are the ones that should be used for long term archives. They should maintain their integrity for at least 10 years when they are cared for properly. The Rewritable versions rely on a transitional plastic layer that can be altered between opaque and transparent (instead of molding bumps). These have a shorter life span which reduces rapidly the more it is written to. These type disks should only be used for short term needs.
Disks should always be handled by their edges without touching either flat surface. The natural oils and acid on human hands can start deterioration of the outside layers. The adhesive on labels can cause the same problem, along with affecting the balance of the disk. Over time they also have a tendency to shrink and may distort the disk’s shape. It’s better not to use any sticky labels at all, but if they’re needed, use ones that cover the whole disk evenly. Store disks away from light (sun especially), humidity and temperature extremes (both hot and cold). They will last longer if they are stored vertically in a case or rack, as opposed to flat.
With proper care the disks will probably last long enough to cover the timeframe in which their archived data might be needed. If you’re storing any contents that might need to be retrieved farther into the future, then you should instigate a schedule to "refresh" the data on new media. By then we may have something truly permanent that you can copy to . .
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