It's because of the pad modules used by FMCB.
I think that FMCB v1.8 uses rom0:PADMAN, which does not support multitaps (At least, I don't remember that it does).
Adding support for multitaps will require some hacking of FMCB because it's a closed-source project.
EDIT: WOW! MY FIRST SUCCESSFUL POST SINCE LAST YEAR!
Finally, people can hear my "voice".
Unmodified SCPH-77006 with SM 3.6
SCPH-39006 with M-chip modchip, SCPH-10281 NA and refurb Seagate 80GB HDD
SCPH-10000 v1.00 with SCPH-10190 PCMCIA NA and SCPH-20400 HDD unit