Install OpenSUSE on MacBook Pro external USB HD

I have a macbook pro with MacOS10.5 and Windows by bootcamp. Now I’m trying to install OpenSUSE on an external USB HD, trying to use Linux on USB_HD. I have installed rEFIt, rEFIt can see it, but when booting into the USB_HD, it always says “GRUB error”. I did install GRUB on /, not MBR.

So what’s the right steps to do it? What did I do wrong?

Thanks a lot.