Installing Windows 10 Bootcamp on 27 iMac (2015): USB 3.0 problem
Actually if you look on Apples website they have updated about bootcamp 6.0.1 on El Capitan.  All 2015 models do not need a USB stick to put Windows 10 ISO in.  All 2015 Mac models from Macbook, Air, Pro, Mac Pro, and iMac do not need USB with Windows 10 ISO.  You can simply download Windows 10 ISO file, put it on desktop, open bootcamp.  Apple website says that all 2015 Macs have the install Mac bootcamp uploader files for Windows within a private partition that uploads after installing Windows and gets to start menu desktop.  If you want to read more of what I just told go to https://support.apple.com/en-us/HT201468  It states right on there that 2015 Macs do not need USB for Windows 10 install on bootcamp anymore.