On my D510MO, I can't boot from a USB2.0 flash device unless "legacy usb support" is enabled in bios. Something to try if you haven't already.
Even then the boot crawls at USB1 speeds until the OS drivers take over for the BIOS. I have a thread about that issue, but no feedback.
I enable the option "legacy usb support" but nothing happen....the screen remain in black waiting for something