I had a hard time figuring out why I couldn't specify an md device in grub as a root device. It turned out that for some reason my kernel does not automatically recognize the md partitions as possible candidates for a raid array rebuild.
This is the normal type of boot:
Apr 12 07:24:02 woodpecker kernel: md: Autodetecting RAID arrays.
Apr 12 07:24:02 woodpecker kernel: md: autorun ...
Apr 12 07:24:02 woodpecker kernel: md: considering hdd10 ...
Apr 12 07:24:02 woodpecker kernel: md: adding hdd10 ...
And this is (about) what I got:
md: Autodetecting RAID arrays.
md: autorun ...
md: ... autorun DONE.
VFS: Cannot open root device "md2" or unknown-block(0,0)
Please append a correct "root=" boot option
Kernel panic: VFS: Unable to mount root fs on unknown-block(0,0)
The solution to this is to add a specific hint to grub to consider the boot partition to be candidates:
# RAID boot
title root (hd0,0) 2.6.6-0 root RAID boot
kernel /vmlinuz-2.6.6-0 root=/dev/md2 md=2,/dev/hda2,/dev/hdc2
See more info in the kernel documentation: /usr/src/linux/Documentation/md.txt
More stuff on RAID that might be helpful here: