Laptop: Dell XPS m1530 with A12 BIOS.
I have USB EMULATION enabled in the BIOS.
At startup, after pressing F12, USB is shown in the list, but I cant boot from it,, boot device gets automatically changed to internal HDD..
Try setting the USB-HDD as the primary boot device.
Also, I want to say that Dell BIOSs treat all bootable harddrives as a single option in the boot order, and then have a secondary list for the boot order of harddrives. You might try modifying the order of that second list if that's the case.