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Well not all USB or DVD's for that matter are 'bootable' and might not show-up in the BIOS. I'd look at your USB DVD OEM site and see if there's a firmware update and/or this is a known limitation. Also, if you have multiple USB ports then try some of the others.
If you're trying to install or repair Windows then often there's a partition on the HDD/SSD with a repair image <or> you can try to make a bootabe USB Flash Drive with Windows; see video - Create Bootable Flash Drive MBR USB installer -> http://www.youtube.com/watch?v=ZcSVVPbzIno
Otherwise please list both your exact Samsung and USB DVD models with links to their OEM sites.