Excerpt from http://www.arrfab.net/blog/?p=11
I don’t like to play with software raid usually (i prefer real hardware raid card on servers ..) but i had to setup a small machine with 2 sata disks in raid 1. The problem is that after the reboot grub will fail to load the kernel (grub error 15).
Solution is to boot in rescue mode , issue a chroot /mnt/sysimage and setup grub manually on each drives :
Reboot the machine and you’ll have grub on each raid device …