It's been about 6 days since your last reply. Don't know if you got it worked out yet but maybe this will help.
I think what stiltskin was referring to was if you install GRUB to your main drive's MBR, which is usually the default. In that case you'll be presented with the GRUB menu every time you boot the machine whether or not the USB drive is plugged in. Grub2 does use UUID so there shouldn't be a problem with consistency. At least there shouldn't be.
But you can install GRUB to the USB drive instead.
When you install the OS you'll have to know what the USB drive is called. The Linux installer may present your internal drive as sda with the first partition being sda1 and if there are other partitions on that drive they'll be sda2 and sda3 etc. If there's a second internal drive it may be sdb (sdb1, sdb2 ect.). You're USB drive may be sdf, first partition sdf1, second partition sdf2 ect. At least that's what my Linux Mint OS identifies my USB drive as.
When you install Linux to the USB drive you can install the GRUB bootloader directly to the USB drive. I'm not sure what installer OpenSuse uses but in the Mint installer look for an advanced button to change where the GRUB loader is installed. By default it may install GRUB to the USB drive if you are installing the OS to USB but I'm not sure now. It may default to the internal drive anyway. You'll have to look for that option. So for example, if your USB drive is called sdf (your's may be different) then tell GRUB to install on sdf (not sdf1 or sdf2, just sdf). That way you don't have to have a GRUB menu when you boot Windows. You just have to plug in the USB drive and tell the computer to boot from USB.
Edited by searchme2, 30 April 2012 - 11:32 PM.