I can't seem to erase re-writable CDs and DVDs with either DVD burner. I have allowed K3B to set up for the first time, open etc, placed in the CD / DVD and selected either 'Erase CD' or 'Erase DVD' depending on which I am doing.
It comes up with an error, and won't erase the CD/DVD. Window's can do it why not K3B ?
My MASTER DVD is dual layer LG model my SLAVE is single layer Iomega. I have tried the same CD / DVD in both drives and get the same result.

There appears to be a structural problem with the infrastructure underlying K3B and possible all other cd/dvd writers on Linux. The kind of issue you are reporting has been affecting me too. On some distros it all works fine, on other ones, not at all. Ubuntu Feisty was hopeless unless I used a SATA dvdrw but now with Gutsy, the problems appear to have been cleared up. I would guess that the heart of the problem is that Linux has recently gone through a major reorganization of the way that hard drives and optical drives are identified by the system. This happened with the introduction of the 2.6.20 kernel and that's when I started noticing these issues. But with 2.6.22 and up, all should be back to normal (although it isn't so far on Fedora 7, which does have 2.6.22, I'll have to wait until F8, I guess). Of course, this doesn't solve your issue but it may be good to know that this is a temporary issue. All you can do for now is wait for an update.