I am unable to reproduce this issue on our side. The unit boots fine from both devices - the USB3 and from the internal SSD.
The setup includes Afaya SSD 32GB and Transcend 8GB USB3
What is your BIOS version?
What is the type of the USB3 device do you use?
I imagine my problem is due to the same issue. I want to put /home on a USB3 disk, but it does not mount in time to be available after booting (though it is available later), so I cannot login.