If you check the claims made by some manufacturers of popular CD-R brands, you will see that some make bold claims indeed. Typical boasts include: "100-years archival life", "guaranteed archival lifespan of more than 100 years" and "one million read cycles". One company even says data can be stored "swiftly and permanently", leaving you free to bequeath those backups of your letter to the electricity company to your great-great-grandchildren.
But an investigation by a Dutch personal computer magazine, PC Active, has shown that some CD-Rs are unreadable in as little as two years, because the dyes in the CD's recording layer fade. These dyes replace the aluminium "pits" of a music CD or CD-Rom, and the laser uses that layer to distinguish 0s from 1s. When the CD is written, the writing laser "burns" the dye, which becomes dark, to represent a "1" while a "0" will be left blank so that if the dye fades, there's no difference; it's just a long string of nothing to the playback laser.In a further article on Digital Preservation from the National Archives of the UK regarding storage media for long-term preservation, their suggestion for CD-R use was limited entirely to CD-Rs with a gold reflective layer and phthalocyanine-based dyes, since recent research suggests that these are the most stable, and have the greatest life span. The FamilyTree article does recommend the gold type CDs but does not explain the volatility of every other type of CD/DVD. What the article does not emphasize is the need to avoid technological obsolescence by verifying and, if necessary, migrating your storage media every year or so. As the FamilyTree article states, "Don't forget to transfer files to new media as they become available." It does no good to have a gold CD if you can't find a CD player that will recognize the disc format. To quote from the UK National Archives, "In situations where multiple copies of data are stored on separate media, it may be advantageous to use different media types for each copy, preferably using different base technologies (for example, magnetic and optical). This reduces the overall technology dependence of the stored data. Where the same type of media is used for multiple copies, different brands or batches should be used in each case in order to minimise the risks of data loss due to problems with specific manufacturers or batches."
The UK National Archives gives a score to various media based on longevity, capacity, viability, obsolescence, cost and susceptibility. Linear Tape Open (LTR) comes out on top as the most desirable media, followed by DVD-R, Hard disks, CD-R and last Flash Drives. But in this world of rapidly changing technology, this article from back in 2008 is itself obsolete. Hard drives are much, much less expensive than they were just two or three years ago.
From my standpoint, the amount of data to be archived determines the type of media. CDs hold relatively small amounts of data and very large files, such as hundreds or thousands of photos, will not fit conveniently on CDs. DVDs hold more data, but with large files or a large number of files, DVDs are still not an option. For example, one backup of my current files would take over 100 DVDs! Think how long this would take to copy. Also, there are currently fifteen different types of writable DVDs. If I had a DVD recorder that would handle the highest capacity DVDs, it would still take over 25 DVDs.
There is really no good stopping place for this discussion. I guess the point of the whole post is to counsel caution in relying on any one type of backup. I presently use hard drives (the last backup took over 5 hours). I have multiple drives and periodically give each of my seven children a copy of the whole backup to keep in different parts of the country.