OS X does not natively support the SWIM floppy controller in pre-iMac Macs. Thus, by default, it cannot read *ANY* disk in an internal floppy (like in a Molar Mac.)

An 800 kB disk will work fine booted into OS 9.

There was a project that ported a MkLinux floppy driver to OS X, but it was very buggy and insanely slow.

Hey, a quick search turns up my writeup on it from five years ago.

USB floppy drives work fine, but only with HD floppies, not 800k Apple floppies.