After having updated to BIOS 25, I have the same issue, however with the D54250WYK.
Prior to updating to BIOS version 0025, I was able to boot from USB. I have tried reverting to BIOS version 0024, however, I can no longer boot from USB devices despite resetting the BIOS settings to their default, however:
Using gdsik, I had zapped the USB device thereby wiping the MBR / GUID Partition Table. I then created a GUID Partition Table with partition type ef00 (EFI) to which it was able to detect the USB device (F10) at boot, however, could not detect the operating system set up on the device (missing operating system error).
I suspect a bug in the BIOS as it remains despite reverting to BIOS version 0024.
Next, I will try reverting to BIOS version 0023 to verify prior statement. Failing this, I will remove the SATA drive and attempt to boot strictly from the USB device. This seemed to solve an earlier problem I had where the display via DisplayPort did not appear after fiddling with the Legacy/UEFI settings.