You can manually install GRUB onto a device if it was installed to the wrong place by issuing
sudo grub-install /dev/sdX
after booting and logging into your Ubuntu installation if that is even possible (as it seems from your question).
However if it's not possible to boot the OS you can use "Rescue a broken system" at the USB drive's startup menu to install GRUB. Just follow the instructions to set up keyboard and timezone and then mount your root partition. Then when asked choose to Execute a shell in /dev/sdXY. If you have created a separate partition which shall be mounted at
/boot you have to mount it manually using that shell (else the installation of GRUB won't work as expected because it will not find your kernel image):
And then install GRUB using:
Then exit the shell (Ctrl+D) and choose reboot from the menu. Now everything should work as expected.
The Ubuntu Desktop Edition provides an advanced configuration option (which lets you set where GRUB is installed to) right before installing the OS. But I couldn't find an equivalent option in the Server Edition's installation process and it seems to always install GRUB to the first disk device. Or maybe I also just overlooked it.