Alright,i understand your point.But the drive is fine and right now i am successfully erasing a written RW with CDRWin which fails in Nero and ImgBurn in full-erasing mode.So i honestly dunno whats going on ...
Edit:
Its software related!The applications (also tested a tool called Active DVD Eraser) only erase a disc in full mode,when its fully written with data.When its quick erased it then can be reburned fully but not erased in full mode!So actually there must be some information stored after quick erase which tells the applications that theres no need to erase the complete disc.