CPC 664 / Amstrad CPC 6128 floppy cable

[Poliander]

I made myself a floppy cable to connect a FD-1 as drive B to my CPC 664 (in order to conserve the internal drive). Works perfectly, except the drive now responds both as drive A and B. After cutting several wires I gave up (surprisingly, the floppy is still working). Could anyone give me a hint what I have to do to use the external drive as B and leave the internal one as drive A?