Just picked up a 128k with a monster Mac conversion, complete with carrying bag, keyboard mouse and system disks.
Checkerboard screen so will clean and recap. Any idea what the black thing is that’s only connected to power on the power board but nowhere else? It’s Velcro’d in ...
I've installed Mac OS 9.2.2 using the image from MacOS9Lives onto 6 mac mini G4s. I only had difficulty with one of them. It seems that the DVD-drive in that machine is defective.
I got around around my problem by attaching an external firewire DVD-burner and booting from a newly burned boot CD. I assume that you put the newly formatted hard-drive back into the mac mini G4. (It's a silly question but I ask since you didn't mention it.)
For all of the machines (including the one that I booted from the external drive), I pressed C at startup and held it there until I saw the familar smiling mac.
Does nothing happen when you press Command-Option-O-F at startup?
I found this site helpful when I was diagnosing problems: https://www.osxbook.com/book/bonus/ancient/whatismacosx/arch_boot.html
This section seems relevant to you:
If Open Firmware fails to find a boot device, a blinking folder is displayed.
Open Firmware then loads a file of type tbxi (ToolBox ROM Image, for historical reasons) from the system partition. Note that this would have been the file called "Mac OS ROM" in the System Folder on Mac OS 9, while OS X loads /System/Library/CoreServices/BootX, which is the bootloader as well. BootX is then executed and Control is then passed to it.
Another has come to mind: your mac mini G4 supports target mode. I've never tried it without an OS but I think it is supported by OF so it may work.