boot – Installing GRUB on a different disc

You can install grub on any drive, but it will only boot from the boot drive.

This is a function of your system firmware. The EFI standard allows you to put the bootloaders for all operating systems in the same EFI partition. If you have multiple EFI partitions, you will need to alter your bios settings to select which one is used. Some bioses only support using the EFI partition on the first disk.

Having said that, the EFI partition typically only contains the bootloader which is very small. You can move the /boot partition (which usually contains the kernel and initrd) to any disk, or sometimes even merge it with the main operating system disk.

The partition is called the EFI partition, but this has nothing to do with where it is mounted. The standard is to mount it at /boot/efi and grub may not install correctly if you mount it at /efi instead.