dimension E 520 and certain drives wont show up

​I'm having a problem when i tryed to add another hard drive. when i opened my E 520 the original hard drive was attached to drive C: and the combo drive was attached to drive E: also when i go to my computer it says cd drive in drive D: but there isnt a a cd drive there. so to add the second hard drive I removed the the combo drive from drive E: and put the second hard drive in its place. I then attached the combo drive to all of the other available drives but my computer is not detecting it. whats going on what should i do ?​