Thank you, Erik, Elsa, Frank and Floyd.
Floyd - in the light of what you and others have said, it's obvious I must beef up my backup regime. However, it seems to me that the extreme measures - that you (and others) find necessary - point to one of the major downsides of our digital age. There is just no reliable solution for accessible long-term data storage. Electronic components have a certain useful life-span before they become unreliable and eventually fail completely. I have not been particularly unlucky, but since I bought my first desktop computer in the late 1980s I have had disk failures, memory-card failures and ram failures. On the other hand, I have two 4-drawer filing cabinets in my office which house my archive of 35mm and medium-format transparencies. Many of these go back over 40 years and have never had any care taken except to keep them in darkness and at a reasonably stable temperature, yet they are apparently as fresh as the day they were shot. Digital manufacturers are providing us with ever more high-capacity storage devices at very low prices. Perhaps they should turn their research in the direction of affordable *reliable* long-term storage.