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An odd thing happened here. I tried what you said ( when I had worked out exactly what you meant) and it worked. However, when the computer asked me if I wanted to do the same again, I said yes, and put in a fresh disc. In response I got "there is no disc in the drive". And I got the same message for the whole pack of discs. All ten of them. They all look the same, they all look alright to me, they have never been used before, though they have been hanging around for a bit.