My PC doesn't see the new hard drive

I plan to install a new HDD. I want to clone the old one onto the new. I have the necessary wires connected (done this before) but when I go to Start>Computer> I see the existing C disk displayed but not the new one. I disconnect the new one and attach an older drive I use for backup. It shows up immediately. I disconnect it and reconnect the new one- nothing! But, when I have the new...