Problem with usb cd/dvd burner

[eric tanguy]

I have an usb cd/dvd burner and i can't make it burning. When i boot with
the burner plugged and on the system see it as cdrom device and i can
mount it. But k3b cdrecord and cdrdao can't see it. When i boot with the
burner off and i switc it on after : i can't mount it as cdrom device but
k3b recognize it. The problem is taht only k3b recognize it but cdrecord
and cdrdao not. So i can't burn anything.
Someone have the same problem or can help me. It will make me crazy!!
Thanxes
Eric