Ubuntu – How to install Grub2 on the first hard drive but Ubuntu on the second

You need to install GRUB to the MBR of both drives, and you need to do it in a way that GRUB considers each disk to be the first disk in the system.

GRUB uses its own enumeration for disks, which is abstracted from what the Linux kernel presents. You can change which device it thinks is the first disk (hd0), by using a "device" line in the grub shell, like so:

device (hd0) /dev/sdb

This tells grub that, for all subsequent commands, treat /dev/sdb as the disk hd0. From here you can complete the installation manually:

device (hd0) /dev/sdb
root (hd0,0)
setup (hd0)

This sets up GRUB on the first partition of the disk it considers to be hd0, which you've just set as /dev/sdb.

I do the same for both /dev/sda and /dev/sdb, just to be sure.

Edited to add: I always found the Gentoo Wiki handy, until I did this often enough to commit it to memory.

Solution is to use a bios_grub partition, which is not the same as the /boot partition.

By default the bios_grub partition is 1MiB, and it must be flagged bios_grub. Mine is the first partition on my disk. If your partition 2 is actually /boot as parted suggests, that would not be correct and you should make another 1MiB partition.

With GPT and GRUB2 the minimum filesystem has three partitions: bios_grub, root, swap. (not perfectly sure swap is required)

Why does grub fail to boot after simply running "grub-install"?

Unknown... You'd think it wouldn't modify anything if it says clearly it cannot embed so it can't work.

Why does it say "file not found"?

/vmlinuz is a symlink that uses the boot partition, and the boot partition is corrupt. The bios_grub code was written on top of its ext3 structure. This probably meant that /boot was not mounted, and the grub files seen there were actually on the root system, which didn't contain the kernel.

Why doesn't grub want to install without this setting I set with parted

A GPT partition table has no space for a bootloader, unlike MBR. So a specific partition must be created to hold the boot code. Before running "grub-install", specify this partition with the command:

    parted /dev/sda set 1 bios_grub on

I thought all I needed was a separate /boot. How does the Ubuntu CD installer install it without the bios_grub setting?

This requirement seems to be all that is needed for the Ubuntu installer, but it creates an unstandard system which is broken easily.

When GRUB says "This GPT partition label has no BIOS Boot Partition", it means the bios_grub partition, not /boot.