Thanks very much, François - you solved my fundamental problem. It's half working. The Mac can see it and they're talking to each other. It ejected a disc quite comfortably when I rebooted and held down the mouse button. It's mounted a disc once. But it isn't reading it. And now I can't get it...
So I've done that - and there's *some* progress. It now appears in System Profiler. But there's a DVD inside it - it happily swallowed it and I can see that the drive has power - but it hasn't mounted it.
Thanks François. The optical drive adapter (green) doesn't seem to have a jumper but the one connect to the SSD hard drive (red) does. Any suggestions? Should I swap them round?
It's among the compatible drives in this iFixit list:
https://www.ifixit.com/Answers/View/302725/What+optical+drives+from+other+Mac+devices+can+be+used+in+replacement#answer302892
I've installed a Sony NEC Optiarc AD-7640A drive inside a G4 Cube (using a 3D-printed bracket). I didn't give much thought to drivers when I bought the thing.
Neither Tiger or Leopard can recognise it. And I can't find any Apple drivers online.
It's one of these, so was running on later Intel...