So, I
technically finally managed to boot OS 9 on the mini from my 512 GB drive! The solution is not ideal: it is underhanded, roundabout and weird, but it works. No OF. (I couldn't boot into it from OF, but all I tried was
0 > boot hd:,\\:tbxi
to boot from the disk, meaning without specifying the partition. Not sure how to specify one, but I know it's possible, sticking the partition number somewhere, somehow.)
Throughout the whole post, I assume the boot issue I have both with the IDE device and the external FireWire drive is that their capacity is past 128 GB, regardless of how you partition them, and that the same issue would be present with USB devices past that capacity, as well.
I'll refer to the >128GB devices as "Big Drives" and to the <=128GB devices as "Small Drives".
What you need is:
- An internal, Big Drive you want to boot from;
- An external, USB/FW Big Drive to help you boot into the internal Big Drive (tested with a 1TB external LaCie FW HDD drive);
- An external, USB/FW Small Drive to help you boot into the external Big Drive (tested with USB, connected to a 40GB HDD with an IDE-to-USB Sabrent device).
The solution to boot from an internal Big Drive was simple: fail to boot OS 9 from another, external big drive that contains a
valid OS 9 installation with the modified Mac mini ROM, and that your only other available drive containing an OS 9 installation is your internal Big Drive. It will boot every time, after the "blinking question mark on diskette" icon flashes the question mark around 10 or so times.
There's an exception to this, however:
you cannot "alt-boot" into the external big drive. It has to be the default drive when powering on, or when restarting. You accomplish this by selecting it on Startup Disk. Just once is enough: if you don't switch away from OS 9, you don't need to touch Startup Disk ever again. Else you repeat the exploit to switch back to OS 9 as default.
There's also another tidbit: it's preferable to boot from USB/FW than CD, because, speed aside, when you reboot after selecting your external Big Drive, you have to remove the CD beforehand, and you can't do this if you are booted from it. Removing it right before rebooting makes the exploit not work, and keeping the CD boots into the CD. If you remove it that way, you have to forcibly shut down the mini and restart (then the exploit works, because the external Big Drive is still pre-selected).
If your external Big Drive is FW like mine and not USB, you have to unplug and plug back the FireWire cable onto the drive (or reconnect power supply on the mini) so that the FireWire device activates and can be mounted again (booting into OS X also reactives it, IIRC, but that takes longer). Turning the mini off and on is not enough. You can replug when the mini is off or on, eitherway works. But it
has to be done.
If your Small Drive is FireWire, you can alt-boot into it, to then select an external Big Drive to boot from with Startup Disk, for the exploit. Disconnect the FW Small Drive when screen goes black, else it may boot from it instead your internal Big Drive.
If your Small Drive is USB, and your external Big Drive is FW, you can alt-boot into your FW Big Drive, which will fail, but then it will boot from the USB device. Reconnect your FW Big Drive and reboot (no need for StartUp Disk). Disconnect the USB Small Drive after screen goes black, else it may boot from it instead your internal Big Drive.
If your Small Drive is USB and so is your Big Drive, then your only choice is to boot into the Small Drive from Open Firmware, or to boot from CD (slow, noisy, wears off laser lens, and a greater pain to use the exploit with).
That concludes the how-to. Now here's some explanation as to why this even works, and some weird things I noticed.
When OS 9 fails to boot from a partition with a valid Mac mini OS 9 installation, it already loaded parts of the modified ROM (perhaps even whole), and perhaps also a few other components, into the RAM. Booting fails mid-way for whatever reason, and it starts searching for other devices in the device tree for one with a valid OS 9 installation to boot from (it's another matter whether all devices are listed or not in the device tree). With the internal drive having a Mac mini OS 9 installation and none other, it boots from there with absolutely zero problems.
The moment I had discovered the ROM, in part or total, is loaded from the first device and that it persists was when I had tried to stick an older modified ROM in the external FireWire device, while the internal SSD drive had the most current modified ROM. The older ROM still used the original Happy Mac logo, while the newest one uses a more colorful, modified one. When it booted from the internal SSD, it showed instead the original Happy Mac icon, which only existed in the FireWire drive (which is by that point unmounted and not even spinning anymore).
But it didn't stop there: the old ROM reported itself not a G4 Cube, but instead a QuickSilver 2002 Mac, and that is what it showed in the Apple System Profiler ("Power Mac G4" as opposed to "Power Mac G4 Cube").
It is mysterious to me why 2 drives that can't boot Mac OS 9 by themselves can boot OS 9 when they work together.
By the way, trying to use the same trick to get OS 9 to boot from my external FW Big Drive didn't work, but I'm not sure if my FW drive had been formatted with Drive Setup in OS 9 or Disk Utility in Tiger (if the latter, I certainly put OS 9 drivers in it, however).
Now, to conclude this whole essay, here are the bare minimum files I needed on my external Big Drive:
https://i.imgur.com/u21ijlm.pngAny file further removed, and the exploit won't work.
And to make booting from my USB Small Drive as quick as humanly possible, here's the bare minimum that is required to cover all situations:
https://i.imgur.com/F6lD6I2.pngAny file further removed, and there will be a problem.
- The useless folders will be recreated anyway, so might as well spare any recreation overhead. Same for the preferences files;
- AppleScript Extension is probably unneeded, but I left it there for future scripting to speed things up further;
- The absence of those specific ATI extensions will cause a system crash mid boot, but only when it boots after a previous, failed OS 9 boot from another device, which is what WILL happen when alt-booting into my FW drive;
- Need the 2 FireWire Extensions to reboot into my FW Big Drive for the exploit;
- Startup Disk won't start without the Text Encoding Converter extension;
- The extensions HID library and USB Device Extension are necessary to prevent error messages that slow you down from popping up after you boot.
I think I covered all that I know on this matter. In conclusion, even booting from USB 1.1 became crazy fast.

For obvious reasons.
Edit: Typo fixes.