The only system failure that would trash both hard disc simultaneously would be a serious power supply failure which personally I have never experienced.
Viruses are also a possible problem but there havent been any "I'm going to overwrite all your hard disc" viruses for a while. Also due to the structure of modern - i.e. big - discs, it taks a lot of **** over the structures on the disc to take to you a situation where data is totally lost.
If possible format your discs using different systems - e.g. one disc FAT32, another NTFS. Destructive viruses tend to work on one system and not another.
Also such viruses attack one disc at once. If you machine locks up unexpectantly with the discs hard on, and you suspect a virus, hit the red button!
Clearly CD'Rs in the cupboard are pretty safe. However, I've known these discs to fail. I agree that CD R/W are less reliable. I have no experience with DVD+-R[/W].
Generally I consider some of the comments about possible failure scenarios here to be near paranoia. Nevertheless I must admit that a dose of paranoia over data integrity might have saved me serious pain in the past!
How about using an external HD via USB or firewire and only having it connected for backup?
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John Allcock