System 6 will not work on the PB180. System 7.1 requires the enabler, as has just been said, and a boot floppy is simple enough to hack. I am sure you know how.

More generally, however, I have a couple of machines that exhibit similar behaviour. The closest in kind is a PowerBook 170, which will sometimes boot and sometimes not. Booting is generally partial. The right system enabler is definitely present.

I think the machine is not giving enough power to the drive, frankly, as failure to boot is accompanied by the drive spinning down, and as the same drive is known to be working, in that it works just fine in another 68k PowerBook.