How Long Do Optical Discs Last?
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So you've finally found a compact, cost-effective means of storing your precious stuff in optical discs. While most of us may not be as concerned about the prospect of long-term storage using optical media since it's either we see tape drives to be much more dependable or we're still caught up in a fast-paced lifestyle that much of our documents need to be just a quickly and easily accessible as well (quite the opposite of long-term storage goals), it's important to be cautious of the pros and cons of relying on optical discs. One important factor here is the lifespan this type of storage provides: how long do optical discs really last, anyway?
The answer is tricky; it is not clear how to predict or prove the guarantee you can expect from these discs. One primary reason: uncertainty. We do not know what can happen to the discs we own, especially if we burned the data onto them ourselves. External factors like accidents, heat, humidity, direct sunlight, sub-standard manufacturing processes, old/damaged burning hardware (lasers, lenses, etc.), and other situations can affect the physical condition of these discs, and consequently the data contained in it. Secondly, not all discs were created in the same way. Materials used vary from one manufacturer to another.
In general, manufactured or pressed discs are more tolerant of damaging conditions than home-burned CD-Rs and the like simply because they differ in the way the data is stored: pressed discs have their data literally pressed onto them, while recordable types use dyes that change color or reflectivity under the heat of a laser.
Now factory-made discs aside, measuring recordable discs' dependability can be attributed to the dye used. Three dyes used right now are phtalocyanine (silver/gold at the bottom of the discs), cyanine (blue/green at the bottom of the discs), and azo (bright blue at the bottom of the discs). Some tests can prove that phtalocyanine is slightly more resistant to light than cyanine [link], while azo seems to be the weakest of the lot. Even so, the formulation of phtalocyanine dyes also vary, so your best bet here is to choose a dependable brand (e.g., one that can claim a hundred year shelf life for your discs). On the other hand, some tests also show that phtalocyanine is much more sensitive about the power that a disc-burner laser must generate to avoid errors. If your CD burner has aged, chances are its laser has degraded as well.
Still on disc-burning hardware, recording speeds can also affect the quality of your burned discs. High-speed burning (rates reaching up to 52X) are less dependable, but we wary of too-slow speeds as well—again, this is linked to the exposure rate the dyes can handle.
Thinking of labeling or marking your discs with ink or stickers? Scratches from either side of the disc, whether it's the data/front side or the label/back side, can both damage the disc data. On a recordable CD, scratching from the back literally and instantly destroys data. On recordable DVDs, data can be found inside and usually fail from the outer portions then inwards. You can thus avoid failures in burning by not using up the whole DVD to make way for errors. If you can help it, avoid using adhesive labels as well as marking pens—they can slowly seep into the dye layers, damaging your data. You can write on the transparent area in the center, since there is no data there.
How about rewritable optical media? They are considered much less reliable than plain CD-R/DVD-R discs. It is advised that you stay away from storing permanent and rare data in RWs, unless perhaps you have backups elsewhere.
Paranoid about the decade-long or century-long guarantees? It's wise to check up on your discs few months to see if your data need re-backing up. If you've got a large collection on discs, start off with your most precious files. It is much better to be self-reliant on preserving one's own documents and memories.
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