Windows 2008 installlation starts from USB stick, but are unable to find the USB memory during installation

I have a brand new server/PC with a motherboard from Gigabyte GA-Z170X-UD3 (rev. 1.0)
Link here: http://www.gigabyte.se/products/product-page.aspx?pid=5499&dl=1&RWD=0#ov 

I am trying to install Windows SBS server 2011, i.e. Win 200 server R2.

I have used https://www.microsoft.com/en-us/download/windows-usb-dvd-download-tool to create an installation USB stick ( 64 GB) from the SBS iso file.

The installation starts without problem from the USB stick. First setup screen appears (choosing language and keyboard layout) and next message is:
"A required CD/DVD drive device driver is missing"    and asking me to install the driver.

If I click on "Browse for driver" I can see only the ram drive created (X:\) and my CD-rom drive (not used here). But the USB stick is not shown. So of course the installation stops here asking for drivers to find the "CD/DVD".

So, the problem does not seem to be a bad iso-file or similar. Either there are some settings in BIOS, or otherwise I need to install some drivers for the USB.

Any suggestions?