no usb boot after installed ubuntu 12.04

no usb boot after installed ubuntu 12.04

I installed ubuntu 12.04 server on a Samsung net book "NP-N100S-E02 from a
usb stick the first time I booted this netbook after purchased it, now I
can't find usb boot options in the Bios setup. Always the boot devices
priority has no more than the STA Toshiba harddisk. Somebody mentioned
that Grub could put itself on the USB MBR, that's why maybe USB BOOT
doesn't exist anymore. I'm asking if anybody confirm what I've been told
or provide me with some advices on how to fix that, I'm really feeling
lost