FMCB works on all Playstation 2 models that contain a boot ROM older than v2.30 (In other words, v2.20 and below). Only those SCPH-90000 series consoles made starting from the 3rd quarter of 2008 (Datecode 8C) will have boot ROM v2.30 and newer.
Originally Posted by Stanliwise
He probably made a normal install, and his fat PS2 is an early console. The older fat PS2s will look for an update like mcX:BREXEC-SYSTEM/osd110.elf, while newer consoles will look for mcX:BREXEC-SYSTEM/osdmain.elf. That's the purpose of the multi-install in FMCB: It's for cross-model support.
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