System Restore-path to system HDD

I installed a new HDD and kept the existing as a slave. Copied content to new HDD and refomated old HDD. New HDD assigned as drive C old as drive D. I cannot switch off system restore from drive D because system restore still shows it as being the system disc. Tried disconnecting D but system restore then shows a cannot operate message. I assume that a registry adjustment is required but I cant find where?