>> It starts loading the first block of data, shows the twirly; but after a
>> while the twirly stops, and the machine hangs.

As you may already find to avoid this situation, but just for FYI,
it will be nice to set

0 > real-base F00000
0 > reset-all

Marco> OF prompt. Is installing a MacOS on another HD the only way to get to the OF
Marco> prompt again?

If you need to do on the screen, the answer is half YES.

Even in that case, have BootVars on your HFS floppy, and 
boot from MacOS CDROM. Run BootVars from floppy. You don't need
to install MacOS if you follow above.

If you have serial connection to the another computer, 
just need to Command + Option + O + F to get OF promt.

To be silly enough to my situation, I did not find the
fact that if I had internal modem plugged in the slot,
I did not get ttya (modem line) OF promt. I had to 
unplug the modem card to get ttya to work for the connection.
---
Makoto Fujiwara,