Bill wrote:

It is writing to the internal hard drive on sda1 . I don't think this is
usb in any form.

Well a Pi doesn't have SATA, so it's probably via another USB-SATA adapter.

Do you mean no CD's are detected, or no CD drives are detected,

boot the Pi, do a "dmesg -c" plug in the USB CD, then do another dmesg,
and see if you get messages about /dev/sdc being detected and used as a
CD drive etc.

The drive seems to be detected as sr0

sounds good.

The caddy opens and closes as expected, but the
drive doesn't spin up
The same thing happens if he plugs it into his PC. The drive is seen but
no music ever plays. I think it has failed.

I'd agree, whether it's worth burning £20 to send it back, or just get
another one for £10, I presume it's a laptop/slim one (don't think I've
ever seen a USB powered desktop full-size one) then again, can't see
many slim 48X only full sized.

I think that is beyond what he and I want to do.

We both really want to find a basic non-usb powered drive.

Is he happy to use a full-size drive? got a spare ATX PSU, just short
green to black one the 20/24 pin plug so the PSU starts up, and power
the drive with a molex lead ... might get fun balancing it all on his
knee though.