iMac superdrive can read bought CDs and DVDs fine, but won't recognise CD ROMs and DVD ROMs.

Thanks again, Old Toad. Terrific info.

Apple tells me there is a Genius Bar in my home town (though I can't see any way of making an appointment).

I guess I'll take it in for inspection, then.

(I'll let you know how I got on)

Thanks. I did. I got a page with an Apple map and identified the nearest store (just six miles form my doorstep). There was no option, on Apple's site, to make an appointment — perhaps a UK thing?

But, clicking the red pin, I was able to find their contact details. So I dropped them an email and will phone tomorrow.

Thanks again for all your help.

Nope — despite presumably having been recognized in order to pass verification.

I then tried it in my laptop and it read fine.

Am I misunderstanding how the verification works?

Eric

Thanks, Old Toad.

I didn't even know they could do a diagnostic check for nothing.

Obviously, this old machine is way out of warranty.

I am indeed able to burn a disc, Old Toad: just not read the results, which still strikes me as fantastically strange.

Here's what I did on Mike's (and Urqhart's) Advice :

1. Dismantled the computer as far as the superdrive, following iFixit instructions recommended by Mike. This was just as easy as Mike had promised (I am eating my words above but can't find any way to cross them out): the screen magnets are great; this machine is well designed and satisfyingly easy to disassemble (down to the superdrive, at any rate).
2. There was quite a lot of dust in the back and in the fans, which I addressed with a vacuum.
3. I disassembled the superdrive itself. That was clean as a whistle inside, and the lens was shiny and flawless as if it had just come from the factory. (Or is that just the lens cover? I didn't touch the small moving box which clearly contained the laser; I suppose I could rub alcohol on that shiny plastic; but it really looked unblemished.)
4. Reassembled all, and plugged in a shop-bought CD — recognised by and copied into iTunes at speed (11x) .
5. Then I plugged in two or three computer-burned CD-ROMs. None were recognised; the motor whirred and stopped twice as before, then the computer declared I'd inserted a blank disk.
6. I inserted a shop-bought film on DVD: this was instantly recognised and DVD-Player began to play it.
7. I then tried several pre-burned DVD-ROMs. Not one was recognised and, after stopping and starting twice, Apple declared the disks blank.
8. Finally, I inserted a genuinely blank DVD-ROM and burned a disk image onto it.

The burning went fine. Having burned, it verified the disk and declared the operation a success.

So here's the thing:

it recognised the blank disk and burned a full DVD image onto it;

it happily read the disk — insofar as it verified the track(s) it had just burned, which I assume means they were read?

it then ejected the disk; when I reinserted it, it declared it to be blank!

I should add that this newly burned disk of course opens and reads fine on my humble old MacBook Pro 2009.

So I took on board Urqhart's comments about contrast being lower on DVD-ROM. But I remain logically stumped that it could read the ROM it was burning perfectly well enough to verify and declare the burn successful — yet it refuses to recognise the self-same disk on reinsertion; whilst MacBook Pro proves the DVD is fine.

Thanks, anyway, all (especially Mike) for giving me confidence to take a screwdriver to the iMac with no glue-guns required.

I'm thinking, at this point, it might be worth tracking down an internal superdrive on eBay or somewhere (perhaps, even, cannibalising the one from my MacBook Pro? — just to see if that will still work fine in an iMac.)