Sorry for the delay!
I've just undertaken the suggestion, but to no avail, it simply refuses certain media, even afterwards.
If I insert a standard CD like Norton,Quark 6 etc. it mounts, despite it taking a while.
The behaviour is exactly the same under 9.
I can therefore only draw the conclusuion that the device is faulty. After all the strange behaviour prompted me to install Apple's 1.0 DVD update for this type of iMac.
Thanks for all your help on this!