mauried:
CD drives draw most of their power from the 12V rail. Only the low level logic uses the 5V rail, so if the drive isn't doing anything , like reading or writing, it wont draw power from the 5V rail , which is what you need.
Come to think of it, since as you say, the motors run from the 12V, it probably makes little difference to the logic current draw whether or not the drive is in fact reading or writing.