>on my G4 Cube and one on the mini itself
Try starting the Mini in "Target Mode" (boot with "T" key held down .
Boot the "Cube" normally, and connect the 2 computers with a FireWire cable.
Use Disk Utility to make sure the Mini HDD has OS 9 drivers installed. If they aren't, it needs to be reformatted with OS 9 drivers option checked.
Insert the CD that you say does mount into the Cube, and just drag the OS 9 System over. Make sure you use one of the methods to "bless" it.
Hope this helps. It's what worked for me.