could you somehow take a picture of the bios screen you're looking at? Dell bios is a little minimalist but should have a boot priority (and possible an advanced boot priority). what i mean by advance is you can set which hdd it boots from as the hdd option in boot and im assuming its either grouping your usb with your hdds or it's changing the boot priority to put the hdd in front of the usb. i personally have mine set to usb by default so everytime i run the computer with my ubuntu live usb it boots from that as opposed to my hdd or ssd. Hope this helps! also just thought of this, there are hotswap docks made my companies like thermaltake than can work through the usb port on your pc if you would want to try file recovery that way?