I'm also using KNOPPIX 3.9, and I had the same problem (cdrecord returning error code 12). A Google search didn't turn up anything usefull. But I found a workaround:

In K3b, choose Configure K3b... in the Settings menu. In the Writing Settings, select the Advanced tab. Select "Manual writing application selection," and click Ok.
Now when you open the Burn dialog, you can select the writing app K3b uses (tab Writing, under Burning Device). Choose "cdrdao" instead of "cdrecord" or "Auto".

This solved all problems for me.

HTH