Thanks a lot for all those suggestions

I reverted jumpers to the original (none = master) with no success.

I then booted from the original OSX partion (as I'd previously copied the OSX boot partition to a USB drive). When booted from that the drive also fails. Again it recognises the optical drive, but whenever I insert a ("retail") disk it shows some activity then spits it straight back out!

I also tried with the original optical drive under OSX and I'd also forgotten that sometimes when I try to insert a disk it will not accept the disk no matter how hard you push.

Original = UJ-845-C 

Replacement = UJ875 - According to System Profiler it's a UJ875S

I googled this and there seems to be issues with this. Perhaps I should have done this before I bought it :-)

I think I'm done with both of these drives now! I've got a 2006 Intel MBP which apparently has a UJ-857. I'll remove that and give it a final try. If that does not work then I think this is going to be a optical free Mac Mini!

I'll report back, but those 2006 MBPs have loads of screws to get into the unit so this may be some time!

Again thanks all for your suggestions!

I'm getting a 404 error when I try any of the downloads on this page:

Yeah, sorry. I haven't informed. We're moving them again to their final place. Will update when it's done and the downloads are back online

Return the IDE adapter to its original state (jumper / no jumper, etc.) and if the System Profiler shows the disk, at least you’ve got that going for you. Do you have any other “retail” created disk to try… to see if that might mount? If that would work then it might all be consequence of not having a properly burned Mac mini installer disk in the first place. Try, try again to burn an installer disk from the downloaded file and at a very slow burn speed, then…. try the traditional install from that disk. IF that doesn’t work…

Boot into Open Firmware with your new installer disc already inserted.

*Holding down the Command + Option + o + f  keys all together.   (That’s o as in otter and f as in frank)

Then once at the prompt, type boot cd:,\\:tbxi and then hit enter.
If it won’t boot from the CD then… then your installer disc / CD is quite possibly crap.

But yes, do try a commercially created CD of any sort.
And even if that doesn’t work, do try another burn of the Mac mini v.9 installer and begin again.  Good luck!

I made my own long time ago, but here's the work of someone else:
https://macintoshgarden.org/apps/apple-cddvd-driver-148-edited-universal-scsi-compatibility

Thanks for digging that up Protools5LEGuy! I've never really understood or used HTDM so I'll be checking that out the next time I 'Fire Up'!!