I can't boot into Ubuntu on a usb hd on my HP laptop

I can't boot into Ubuntu on a usb hd on my HP laptop

I used the windows installer to put Ubuntu on a usb hd . It will boot on
my HP desk top ok but it won't boot on my compac NC8230 laptop. I have the
boot menu set to usb hard drive first then other devices when I plug in my
ubuntu and start the computer I get error: unknown filesystem then below
that grub rescue>. Does anyone know how to get it to boot on my laptop.I
want to try a linux based OS.
Ps. I have no trouble booting a portable version of win7 on win8 to go on
the laptop.