Dell Latitude E7470

I purchased new notebook , my notebook come with its Win7 64 Bit Operating System , i want to downgrade win 7 32 Bit , I go to Bios change ACHI mode m Boot Sequency also Enable. I used My working external DVD Drive , but when i tro to but my , USB dvd not detected. but when i press F12 and go to boot option there is CD/DVD After i Press it display press any key ..i go forward as normally,... after  can not see the hardrive.