Booting from an external USB HD
 

I just finished installing a copy of 14.04 unto an external USB drive and all I get when I boot from the disk is the following message...

When installing I selected to install GRUB to /dev/sdb (which is my USB drive). Any help will be appreciated.

It is better to use UUIDs or labels for external devices over /dev entries because /dev entries can change at boot up.

However, in your case, you will likely find that your grub config file is trying to boot from /dev/sdb when it is actually booting from /dev/sda, the first disk found by the BIOS.

The remedy is to change sdb to sda in your grub config file.