OK well i call it a disc lock but i have one of these

and i pass a chain through the frame and something solid..
wether it does much of a job or not i don't know but my insurance said it gave a better discount than an alarm..
I know if someone wanted to steal it that bad they would but my ethic is to make it more of a hassle to steal than say another bike near by. i think this is why they are referred to as "deterrents"