Comment 4 for bug 506670

The problem happens as a result of the drives being GPT which usually means >=2TB. GRUB cannot yet handle GPT boot.
It is more than a documentation bug, we need GRUB updated to provide a way to boot off RAID GPT drives as there will be more and more GPT drives as time goes on. Folks will continue to want to boot off these in RAID config.
Here's the scoop on GPT well described by IBM:
http://www.ibm.com/developerworks/linux/library/l-gpt/

So we need for the GRUB folks to come up with a solution. I'm guessing that they are thinking hard about this right now figuring out what will be the next generation.

Meanwhile a workaround for folks to use while waiting for GRUB to catch up is to do a manual mirror of the boot partition. One way is /boot on one drive and /mnt/boot on the other with both drives marked bootable. Keep them mirrored with a script to copy /boot to /mnt/boot wenever /boot is updated.