In all honesty I was suprised at the questions on the ASTB. The test seemed more like a basic knowledge test like the SAT or ACT. The mechanical reasoning was the only portion that a technical major may have an advantage on, but only for a few questions (electrical resistor network, gear problems, etc.), but I guess most of the problems seemed like simple machine problems. Most of the test seemed to gauge if you have a basic understanding of concepts. I am a technical major (Electrical Engineering), and for me it was more difficult to go back and try and remember things I had learned long ago. I did take the test about 8 months ago however, so I don't remember much else. You're not really at a disadvantage as long as you study the practice guides. I thought the ARCO book was fairly helpful, and if you're going for SNA or SNFO definitely be prepared for the spatial apperception section.
Good Luck