Im trying to install windows on my Mac using bootcamp i have a windows 7 ISO file and a USB stick which im trying to boot from I have but when installed the windows installer onto the USB but when Bootcamp Assistant in the process of partitioning the drive it just shuts down and re boots my mac and i get a black screen telling me no bootable device, also tried holding alt at startup and booting the usb that way but its no recognised any help appreciated.