It was in fact the cable. New cable, all has worked so far. I am currently copying fids from old-drive-1 (30GB) to new-drive (100GB), via the
cp -auvfx /drive1/fids /drive0/ command.

I have the impression it is faster that it would be via ethernet (is it supposed to? What is the theoretical transfer speed via ide channel?), which is really good. In an hour I should be done and move on to old-drive-2 (still 30GB) -> new-drive. I've upgraded a few times in the past, but for some reason I had always used ethernet. Again, thanks Tony for another good suggestion on The Guide

I just realized that since I will be using ONE drive only, the part of the cable scratching the display card is not going to be "used" (the scratch was very close to the ide connector for the slave drive), and therefore I should be able to fold it in a way that it is preserved intact, far from the display card.