I had the same problem, you will figure this out,
And yes I am running SMOS from a Bootable usb stick, plugged into one of the USB 3.0 slots on the back, (you can try others)
BUT, first make sure you wrote the bootable SMOS properly to the USB by powering off a laptop or some other computer, inserting the USB and starting the machine up, and see if Linux starts the boot sequence, if you know your USB is good, then,
In your Bios of the B250 Mining expert, under the advanced tab you should find a BOOT tab at the top, under that, you need to turn off Fast Boot and make it regular boot, and then change from the EUFI Boot to Legacy, not the dual option, but the Legacy only, then save the BIOS changes and power off completely, before powering back on, as long as you have a bootable linux USB drive inserted, it should boot, if you mess up any settings, you can always short the BIOS pins on the mother board to bring it back to default, so no worries, and I did upgrade my Bios to the 12/15/2017 version via downloading it onto a completely different USB and flashing the Bios from within the BIOS,
Hopefully this helps, let me know, if it comes down to it, I guess I could go reboot my miner and take a picture of the actual Bios Boot screen settings,
Good Luck!
Cheers I'll give it a shot. Thanks for your help