I don't remember, it was too long ago 
Checked the boot order ?
You could try what @erizan did in Now 250€, EU eBay new Dell Edge 680 16 core C3958, 32GB RAM 240GB SSD 6x1GbE 2xSFP+ - #15 by erizan, might work for the thumb drive too.
I have no idea what this option does.