#16 Only beep on start

Starting with inserted SD-Card and USB dongle - both with the 2 GB image (it needs 2.1 GB ;-) )- and pressing CTRL + U my Asus is only making a deep beep and show's me the following: > Welcome to developer mode! > Usefull key combinations: - ... - CTRL + U: Boot from external media - ... > This screen is shown for 3 seconds (if not held). > Warning: this device will boot kernels without verifying their signature! > Warning: this device will boot from external media! > Default boot medium: internal storage > Active firmware id: libreboot-r20160907 (RO) What can I do to get my Asus to run Devuan?