Hi Odaman,
If you are NOT plagued by annoying requesters asking you to register your copy of IDEfix, then you DO have a registered version. If your version is unregistered, you will know it.
It sounds very frustrating - obviously the CD rom IS compatible (since it registered on the first IDE line). Have you tried a different IDE cable on the second line, or swapping the IDE cables to make sure it is not acable fault.
btw - is the 'express module' included with your idefix express adapter - the one which connects to the clockport?
Are you connecting the correct 44-pin header to the motherboard? AFAIK it should be the one closest to the edge of the board, but I haven't got a idefix-express so I'm not sure.
Good luck getting it working.