The issue was solved a bit by chance. We went into the BIOS to see whether DMA could not be forced from there. As it turned out, we could not since we were faced with a Department Store Tower, nevertheless with a Gigabyte MB and AMI BIOS, 5 or 6 years old (Pentium 4, 1.7 Ghz).

It's at that point, in the BIOS, that we noticed that the two disks were set to User-defined. When we changed this to Auto, the problem was solved and everything was back to normal as it was before we ran into the defective connector issue which started all.

BTW, is there really that much of an improvement in throughput when you mix dvd reader and disk? Is it simply because you have the 2 disks on different controllers?

Thnaks to all for the suggestions which put us on the right track.