Insert a USB flash drive into a running computer. Open a Command Prompt window as an administrator. Type diskpart. In the new command line window that opens, to determine the USB flash drive number or drive letter, at the command prompt, type list disk, and then click ENTER.
Follow these Step-By-Step instructions from Microsoft (Win10) to make you USB bootable and format it as FAT32 regardless of size. http://www.wikihow.com/Make-a-USB-Bootable