Why duct tape it? If your warranty says nothing about opening up the case yourself, I suggest you open it up and simply unplug the power cable leading to that disk drive. Better yet, ask PC World if they can do that so there are no problems with warranty.
As to why it shattered? Most likely a tiny fracture. A CD undergoes a lot of stress when its being spun due to the centrifugal force. A small crack, one that would otherwise not affect the cd at all would become greatly exacerbated possibly causing the cd to 'explode' in the drive. Its too late to tell where the crack was now, but my money is on the inside plastic ring having a small one ( the clear ring around the hole of the cd ). A crack there would grow outwards until it became big enough to make the cd explode.
