We need more answers. Do you have both the OS HDD (hard drive) and the external hard drive connected at the same time or are you trying to connect the external HDD by itself? If you have tried both at the same time then, the computer might not support having 2 hard drives of different brands. At work I was trying to do this, with 2 HDD's of same type of computer, but both HDD's are different brand and the computer would not boot because of the different type of brand. I was using a Maxtor and a Western Digital.
Another thing, you might have to put the external drive as Slave, maybe the computer is trying to use both as master.