Can't Erase CDRW

[zetor]

I have a PX-504A which has been working fine until now when I try and erase a CDRW, I am using Nero 6 and it tells me that it isn't a RW, it has done it with 2 discs now and both had been preveiously used in this drive, any idea's most welcome.
Zetor

 

[roadworker]

Possibly an illegal toc.....try to erase them with Alcohol 120% options "full erase" and "ignore illegal toc" selected.