[Bug 25380] solution for me:

[aexl]

i had exactly the same problem. eveything worked fine after i added the following setting in k3b:
options > Programs > userdefined-parameters > cdrecord
dev=/dev/sg1:1,0,0

background:
the key was the line:
>Error trying to open /dev/sg0 exclusively (Permission denied)
(where sg0 is NOT the CDRW but my hard drive which has permissions 660 root.root)
k3b calls cdrecord with parameter dev=1,0,0
i assume(!) that cdrecord does a scanbus  to guess the device. which as non-root gives me said error.

if k3b specified the device node as i did the issue would be solved. 
so imho this really is a K3b issue.

-- 
No way to select cd-recorder device
https://launchpad.net/bugs/25380