Can i put 2 hard drives in my laptop, if i have a "C" and "D" drive?

You do not have 2 drives. A laptop can only ever have 1 physical drive. You have a single drive, partitioned into 2. Laptop drives are totally different to normal drives. You need a proper laptop drive. You would need to find cloning software for the copy over of your system to one partition on your bigger drive, this will need to be capable of working on a USB drive. Not many of them do. You would then need tocopy over the D: drive files onto the second partition of the external drive and then fit the drive into the laptop. Make sure your cloning software actually does make the new drive bootable.