Aurora R7 - how to install windows from USB

I feel pretty stupid asking this as I've been able to install windows from USB simply by inserting the USB into the computer, so am guessing I have changed some setting in the BIOS but can't figure out what. When I power up, the USB stick lights up indicating it is being read/accessed by the computer but it still boots up from Hard disk (two SSD in RAID configuration) - it used to read the USB and install windows.