Not to worry. After days, trying to get Windows 7 working, on my NUC6i5SYH. I have given up.
I have now installed Windows 10 Pro. Not a single problem! Keyboard and Mouse worked during setup. Missing drivers installed using Intel's Support page, see here Drivers & Software.
To all thinking of attempting to install Windows 7 on the NUC6i5SYH or (NUC6ixXXX) do not bother. Just choose one of the following;-
a) Buy one with Windows 7 already installed.
b) Buy this as you will be able to use the 1.25mm header on the circuit board and use your USB keyboard and mouse. NUC Lid Additional Dual USB 2 0 Ports | eBay
Seeing the number of posts on the subject of slipstreaming the USB3 drivers onto an USB install image, it's pretty much hit and miss.
Too many variables that can go wrong. Heck I even saw a post of someone who spent time to no avail, only to find that when he tried to do the procedure on a different PC, it worked a charm
Often people own a copy of Win 7 and want to use that (and perhaps later do a free upgrade to Win 10) instead of dishing out the $$$ for a Windows 10 license.
This message was posted on behalf of Intel Corporation
If you want to install Windows® 7 on 6th gen NUCs, it is necessary to follow the instructions below:
Windows 7* USB 3.0 Creator Utility