Setup grub2 on a GPT software raid 3 TB HDD

Even with GPT you should still be able to install GRUB on the drive (/dev/sda?) Try these commands:

# grub_bios-install --boot-directory=/boot --no-floppy --recheck --debug /dev/sda
# grub-mkconfig -o /boot/grub/grub.cfg

it should make you a working GRUB install. You will however probably need to tune some options in the GRUB config files to get grub the way you like it.

On legacy BIOS machines you need a "BIOS boot partition" which may be as small as 30kb on the drive, this is required to store stage 2 of the GRUB boot process.

To create this BIOS boot partition do the following:

A BIOS Boot partition can be created using any of several different disk utilities:

In GParted, create a partition (an unformatted one works fine), then right-click it and select Manage Flags. Check the bios_grub flag, click Close, and apply your changes.

In GNU Parted (parted), create a partition, then type set 1 bios_grub on, changing 1 to the number of the partition you want to mark as a BIOS Boot partition.

In GPT fdisk (gdisk), create a partition and give it a type code of EF02.

Source: http://en.wikipedia.org/wiki/BIOS_Boot_Partition_%28GPT%29