GRUB error 21 --- USB HDD

I am having a problem in that when GRUB loads, my USB HDD in which openSUSE (and any other distro) is installed, turns off. This leads to error 21. Apart from making a seperate /boot on the internal drive, how could I get it to work. This is pretty much all of the info that's neccessary, I know that this is the problem

I googled out this link: might solve your issye.
Check this Grub Error 21 after full install to USB hard drive | USB Pen Drive Linux