I have a advent pc. i have a advent pc pentium d 3gb ram with 2x 200gb internal hard drives one

Question

i have a advent pc pentium d 3gb ram with 2x 200gb internal hard drives one with the letter c the other letter d when i turn on my iomega 500gb usb external hard drive my internal hard drive letter d disappears and is replaced with the external one as letter d and no sign of the other internal i am running vista home premium with sp1 any ideas?