Harddisk conflick

I have 1 harddisk partition into half. Now I can see c: and D: ,and my cd-rom will be E: ,the problem is when i put another harddisk with also half partitioned, it should be E: and F: and cd-rom G: ,but it turn out to be D: and F: , and the first hardddisk become C: and E: , so all the program corrupted or can’t be found because of the drive letter. I have tried to change the letter assignment from the device manager but can’t be done. Can’t some one please help…