You need to create a UEFI specific boot USB drive to boot usb devices with a UEFI system. Some Dell models (like my new Inspiron 11) doesn't even have a legacy mode.
1) Disable UEFI secure boot (this is a key exchange that occurs for an installed system)
2) Create a UEFI enabled Windows OR Linux usb drive (MUST be formatted FAT16 or FAT32):
Unbuntu: https://help.ubuntu.com/community/UEFI
Windows 8: http://www.eightforums.com/tutorials/15458-uefi-bootable-usb-flash-drive-create-windows.html
3) When you boot, press F12. If done correctly, it SHOULD show up as a bootable device.
Just went through this crap tonight and randomly answering forums in hopes of saving other people from this headache!
Goodluck!