Note that the later A9M0106 "universal" 3.5" drive that's styled like an SE/Apple IIgs works fine without a newer ROM or the HD-20 init; I have one of those myself and can confirm my non-"e" 512k can boot from it.  (Presumably it has a fix to the TACH line similar to that suggested in BMoW's blog entry.) The original non-"Snow White" external does indeed produce the same divide by zero error if you try booting it or using it without the init. (Have one of those too and have observed the same symptom.) So... if you have a IIgs drive handy you can try it.

(NOTE! Don't confuse this drive with either the "Unidisk" drive or the Superdrive version; I know for a fact the Unidisk one won't work and may well damage your machine, I'm not certain about the Superdrive version.)