One of the drives in my RAID 10 array failed during a power outage, putting the array into a degraded state. Unfortunately, it was the first drive in the array, which contained the bootloader. I was able to use an Ubuntu Server install CD to boot the server into rescue mode. When the rescue procedure prompted me to select a root device, I pressed Ctrl-Alt-F2 and switched to a shell instead. I was able to partition the replacement drive, add it to the array, and rebuild the array, according to this guide:

http://stackful-dev.com/raid-101-rec...rives-die.html

So far, so good. However, when I try to repair the bootloader, I cannot mount the array (/dev/md127) because it says "device or resource busy." I've also tried to stop the array but I see the same error message. If I try to reboot the server without the Ubuntu CD, it goes to a GRUB prompt and can't find the root device. What else can I try to repair the bootloader?

Here are the details for my server:
Ubuntu Server 12.04
RAID 10
/dev/sda hd0 (sda is the replaced drive)
/dev/sdb hd1
/dev/sdc hd2
/dev/sdd hd3
/dev/sde hd4
/dev/sdf hd5

* Please note the partitions in this array are GPT because the drives are 3 TB each. I read that you can't simply use dd to copy the MBR because of the unique IDs of the drives. Each drive has a BIOS boot partition and a data partition:

Is this an efi system? What is the ouput of at the grub prompt? You can use at the grub prompt to search for your grub.cfg file. then something similar to this to get menu. I'm assuming your not getting a grub-rescue prompt.

grub> set
color_highlight=black/white
color_normal=white/black
pager=
prefix=(vg_raid10-root)/boot/grub
root=vg_raid10-root

I ran
and it booted! May I buy you a beer, sir?