Yes, I use usb 2 ports and I tried all usb ports without succes and the usb key is the one I used to install El capitan first.
I ve tried a unibeast key formated as MBR but no change.
Bios setting is OK:
Load the Optimized Defaults (F7); I did it
M.I.T. > Advanced Memory Settings > X.M.P. > Profile 1
BIOS Features > Boot Options Priorities <--- set this to the UEFI !!!! NOT POSSIBLE FOR ME !!!
BIOS Features > VT-d > Disable
BIOS Features > Windows Features > Other OS
BIOS Features > Boot Mode Selection > UEFI only
Peripherals > Initial Display Output > IGFX
Peripherals > Intel Processor Graphics > Enable
Peripherals > XHCI Mode > Auto
Peripherals > EHCI Hand-off > Enable
Peripherals > XHCI Hand-off > Enable