If you'll look at the first step in part XI, you'll see that we do "grub-install". From its output, we can see that on my development box HD0 is the first IDE drive and my usual system drive, and we can see that HD1 is the only other drive detected.
Then in the last grub step, you will see that we install to HD1 and not HD0 so the base system will remain unchanged.
If you do accidentally overwrite the grub installed on your base system drive, simply boot to any rescue media to fix it, the rest of the system will still be intact.
re: how to not fsck up existing system on step XI (grub-stuff)
If you'll look at the first step in part XI, you'll see that we do "grub-install". From its output, we can see that on my development box HD0 is the first IDE drive and my usual system drive, and we can see that HD1 is the only other drive detected.
Then in the last grub step, you will see that we install to HD1 and not HD0 so the base system will remain unchanged.
If you do accidentally overwrite the grub installed on your base system drive, simply boot to any rescue media to fix it, the rest of the system will still be intact.