After the entire disc image from the source disc is written it ejects. I don't know, it's always done this and I didn't set it up to do that, not to my knowledge anyway. I used to only have one drive, an internal burner which burnt cds only, so it wasn't a problem and actually the ejection of the source disc was needed so i could put the blank in there to be burnt on the same drive. But now that I have an external dvd burner this is becoming quite a significant problem, as the automatic ejection and re-entering of the source disc uses computer's resources during the burn and interupts the process.
The burns don't completely mess up, they finish "sucessfully," but clearly this interferes with the process & also degrades the quality of the burns as scans seem to indicate - As opposed to scans of discs I've burnt when I've been there to snatch the source disc out of the source drive, before the drive re-enters it & reads it after a few moments.