Next week I will be getting a new Samsung 840 pro 256Gbyte ssd. I am going to do a clean install on it. According to the installation of this new drive I have to unplug the old drive that has my operating system on it and plug in the new drive so it is the only drive that will be seen by the bios so I can do a clean install. My question is what happens when I plug in the old drive that still has the operating system on it? Will there be confusion as to which drive my computer will boot to? My old drive has to much on it to fit on the new ssd drive. My old drive is a 3Tbyte drive with 352Gbyte used up space. These are mostly games.