Although you can copy a plot file from one drive to another, you will only be creating a duplicate plot file and that will not increase your mining capacity. Instead, you will end up with two overlapping plots and most mining software will complain if you try to mine with overlapping plots.
You can think of each nonce in your plot file as a lottery ticket. If you had two lottery tickets with the same numbers on each ticket, then your chances of winning would be the same as only having one ticket. Likewise, you will have to plot each of your drives in order to create more unique lottery tickets (nonces) to increase your chances of winning each round.