The safest way to protect your files is to back them up to both a hard drive RAID and an optical disc. The typical magnetic hard drive is warrantied for three years. It contains moving parts. The more it's used, the less life it has remaining until it fails, and all hard drives fail eventually. Using a RAID to back up your files is going to be much more secure than a single drive, because you won't lose your files when one drive in the RAID fails.
On the other hand, the life expectancy of optical media, i.e., CD, DVD and Blu-Ray is not affected by how often the data is read, but by the stability of the dyes in the data layer and how the discs are handled and stored. TDK, for example, rates the life expectancy of their Blu-Ray media at 50 years. That's an ideal scenario, and it will be affected by how the media is stored. All optical media should be stored in a dark environment with as little exposure to air and excessive heat as possible. With CDs, there is some evidence that those with a gold data layer will survive longer than those with more typical dyes. Of course, scratches can render any optical disc useless, so it's crucial to handle them carefully and not leave naked discs lying around when not in use. In all cases, you should stick with a well-known brand and avoid proprietary store brands and discs made by any company who's primary business isn't making recordable media. For my money, you cannot buy a more reliable brand than TDK. I've also had good experiences with Maxell, Memorex and Verbatim.
