Yes, although they are treated in about the same way.
What if the first partition is active? Does it jump from the first of the disk to the first of the partition which may not be that far away?
Is the first sector of the partition reserved?
Yes, for some meanings of “reserved”. It is reserved for the operating system to us for booting, so it cannot be part of an ordinary data file.
So how do I get code into the first sector of the partition and that is the only thing that is done?
Normally, by installing a boot loader. The command “grub2-install” does that.
I just feel I need to understand this so when things go wrong, I don’t mess up the menu or other things.
That’s going to depend on which boot loader you use. With “grub2”, installing it will rebuild the menu.
When I have both the external drive and my hard disk at the same time, if I use YaST boot loader, it will create a menu of everything. I want the menu left alone (and not meaning to uncheck the probe foreign os).
When you boot the external drive, it will use a different menu (if you set it up properly). You want it to use the menu on the external drive, rather that the menu on the internal hard drive.
I may not remember exactly what I did. But there’s an option for Boot loader location to be “Custom Boot Partition”. I expect if I say my external disk, that will overwrite the menu contained on it.
Unless you have multiple operating systems on the external drive, it is best to install grub2 to the MBR of the external drive.
To do that, you need to set the disk order to be assumed by grub2, so that the external drive comes first.
Maybe I haven’t seen OpenSUSE’s wiki. I found one about grub and 11.1, but it doesn’t seem to answer my questions.
If you want full directions, we can give them. But it isn’t clear what you want at this stage, so it’s hard to give directions until you are clearer.