Actually, I don't want the cold cathode light to turn on when the Hard Drive LED turns on, I want the cold cathode light to turn off when the hard drive LED turns on. What I was thinking was that when the HD LED turns on it would switch the transistor on. Since the path through the transistor would offer less resistance than the path through the cold cathode light, the current would bypass the light. Then when the HD LED turned off, the transistor would turn off and the current would have to go through the cold cathode.
When I tried setting this up, the Cold Cathode light just stayed on no matter what the HD LED did.
As a side note, I did try putting the cold cathode light in series with the transistor just to see if I could get the transistor to do anything, and it did sortof work, but when the HD LED turned on and the transistor turned on the cold cathode light never did get as bright as it does when just plugged into the computer normally. And when the transistor turned off, the cold cathode was still on, even though it was much dimmer than it was when the transistor was on.