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Over the years, I've occasionally had to retrieve data from backups I've made to CD or DVD.

All these backup media were burned with extra care (at low speeds, not always with high-end media but never with the cheapest crap either) and almost always with the burning program's double-check option turned on.

However, every time I had to search through some CDs or DVDs a couple of years later, there were shockingly many occurrences of data corruption on a shockingly high number of the media.

I managed to work around the corruptions so no serious damage was done (as they would usually span only across a few sectors, or whatever they are called on CDs/dvds), but is this a normal rate of decay for CDs/DVDs? Does the storage method influence the media's longevity? I usually store them in soft plastic pouches. Could chemicals permeating from those pouches be the problem?

  • Is there a way to prevent decay of CDs/DVDs? Is this a brand issue, with cheap media decaying faster?
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I'm not completely sure whether this is on topic on SU in this form - if you think it isn't, and think this can be improved, please let me know! –  Pekka 웃 Jun 3 '12 at 16:57
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Yes, it is normal. You cannot do much more than storing them in a cool dark dry place and copying the data to fresh disks every 3-5 years –  Akash Jun 3 '12 at 17:01
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@Pekka: Your two questions (about CDs and cloud storage) are pretty unrelated, and both were discussed here on su.com... For example: superuser.com/questions/251369/… (there are other similar topics) –  haimg Jun 3 '12 at 17:03
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@avirk if you are using it for a decade you have a better chance of it working than if you used it, kept it away for a decade and came back.Same for SSD's, the data decays after 1-10 years depending on the type of cell used in it –  Akash Jun 3 '12 at 17:09
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@Akash roger that Sir. ;) –  avirk Jun 3 '12 at 17:10
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2 Answers 2

Probably preaching to the choir here, but this is experience gained after a couple decades of using the darn things. My first drive was a really expensive SCSI interface 1x CD-R. That $250 you pay for a really good manufacturer now? Pfft! I could buy six now for what I paid for that thing. Ouch!

  1. Use a quality drive to produce the disk
  2. Use a quality disk manufacturer (Mitsubishi, JVC, Sony) and as fresh a batch of disks as you can get.
  3. Make sure the disk surface is clean before inserting into the drive
  4. Slower burn speed is supposed to produce larger dots, verify data after burn
  5. Multiple copies of important data is a "Really Good Idea™"

Also:

  1. Time will degrade the write-ability of the dye layer, slow burn speeds will not make up for this, use as fresh a disk as you can get.
  2. Heat will degrade the dye used in the writable layer.
  3. Ultraviolet light will degrade the dye used in the writable layer.
  4. Moisture can get through the label side coating and attack the aluminum flash layer.
  5. In certain environments, there is a sort of mold that will eat the label side of the disk, even on pressed disks.

The 20-30 year lifespan quoted for writable disk life was based on advanced aging tests, not real lifetime tests. Real life shows that a high proportion of written disks probably aren't much good after 10 years.

So the name of the game is "make multiple copies over several media types for long term storage" on rather static data. With multiple copies, there's a chance of recovery of the bad files.

The case is not so good if large databases are involved, a couple bad spots can render the whole database useless on a couple disks. Most backups have a lifetime of weeks, so this isn't as much of an issue, but for monthly backups, you may need to do refresh copies every so often if long term storage is a legal liability.

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I wonder what this means for nuclear waste repositories that are supposed to last 100,000+ years. :) Good points, thanks! –  Pekka 웃 Jun 3 '12 at 18:10
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The Latin saying is "Cum Grano Salis". "With a grain of salt." Or as my Grandfather said about free advice, "That and 50 cents will buy you a cup of coffee." which was true in the days of 50 cent cups of coffee... –  Fiasco Labs Jun 3 '12 at 18:14
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Use DVDisaster to mitigate the CD/DVD deterioration problem. Remember, I said mitigate, not solve.

If you put some effort, and regularly check the integrity of your disks, DVDisaster will inform you of the extent of the deterioration, if any, and then can actually repair it! Once repaired, you can write the disk's data onto a new disk. Thereafter you should continue to check the new disk for deterioration regularly ( I think checking every year should be more than sufficient).

DVDisaster, combined with the other advice given by Fiasco Labs is a very good (and cheap) archival method.

One (small) disadvantage of DVDisaster is that it requires space on the disk to store recovery data (Reed-Solomon code). The more space you allow for recovery data, the higher the probability of a successful recovery. So, if you allow more space for recovery data on the disk, you can do integrity checks less frequently.

This has an advantage over manually making multiple copies of the disk: it is more convenient to just command DVDisaster to repair the deterioration using recovery data, than to manually go through the disk's files yourself to see which have succumbed to deterioration, and then, go look for their copies on other disks and manually copy them over, and slowly recover the original disk. Also, we cannot be sure that the survivors don't have a teeny tiny deterioration in them that makes them appear unharmed: those files can be opened and read, without any application errors.

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