When I boot Ubuntu from my USB hard drive, it says "Missing Boot MNGR". It only boots from CDs and DVDs.
This is an Acer Aspire with Windows 7 boot manager.
It sounds like your system is trying to boot off the USB hard drive's boot sector but the hard drive doesn't have grub or another boot manager installed. Have you tried using super grub disk (it's a downloadable for a CD which should work well) to try and boot from there?
link for SGD: http://www.supergrubdisk.org/
Have you checked out this guide?
Seems pretty easy, however it looks like you need Windows to run that utility initially. From the looks of it, you have Windows so I don't think this will be a problem.