boot – iBook G4: black screen when booting with USB plugged in

I have an iBook G4 with Tiger on it, and I thought I would try out dual-booting with Lubuntu just for fun.

I downloaded Lubuntu 14 and prepared a USB 2 stick to boot from using dd. So far, so good.

The problem is that the machine only shows a black screen on boot if the USB is plugged in. This is both when booting normally and when using the Open Firmware key combo. It starts, caps lock lights up when pressed, but nothing happens.

Without USB drive connected, both normal boot and OF works. Once booted, USB drives work normally.

The optical drive just spits out all media, so I guess that’s broken and cannot be used to boot from.

After searching for a while, I haven’t seen anyone else describing this exact problem.

Any ideas?