Why must "device for bootloader" be /dev/sda instead of /dev/sda1 or /dev/sda2?

Is it that GRUB can't be installed on a partition?

How does BIOS recognize the bootable devices? Is it that installation writes into BIOS?

  • If a BIOS based system, BIOS boots from the MBR, or very first sector of a hard drive. It cannot directly read a partition. New UEFI systems boot from an ESP - efi system partition. Parts of grub are in boot loader location, parts may be hidden and grub menu is in your /boot/grub/grub.cfg. gnu.org/software/grub/manual/grub/grub.html#Introduction – oldfred Nov 27 '18 at 16:39

In BIOS mode, the computer system looks for a bootloader in the very head end of the computer, which is pointed to by /dev/sdx, while /dev/sdxn points to the head end of partition n in drive x.

If you have a bootloader in very head end of the computer (MBR), you can chainload to the head end of a partition (PBR), and in this case it can be useful to install a (second) bootloader there.

If you do not want to overwrite the head end of a partition (MBR), you can write to the head end of partition (PBR) without chainloading. Later on you can run sudo update-grub in the main operating system and get grub menuentries for the new (dual- or multi-boot) operating system.

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