Why when I unchecked DMA in Windows 7, the CPU usage was still very low?
Your understanding of DMA is OK. But disk accesses are generally slow. So the CPU may spend is time on waiting for the disk to write the bytes on the disk, he is not stressed.
May be you can try to run something like CPU Burner that will stress the CPU and then measure the time to copy with/without DMA enabled.
Note that many things can interfere with your measurements, particularly HD caches.