Boot Camp - Intall windows on a external sd card
I'm trying to use boot camp to crate a windows 10 boot usb drive, but boot camp only let me instal on the local hard drive. I've only have 256Gb but i don't have space for a new partition so I bought a 200Gb sd card to use windows. On my older macbook i could create a install disk to boot from it. On this new one don't have the option of creating a usb.
How ca i do this? i need to install windows 10 urgently.
Thanks
MacBook Pro with Retina display, OS X El Capitan (10.11.1)