I have installed Ubuntu 12.04.1 LTS AMD64 on an old Dell D630 with 2x 1 TB HDDs in a RAID1 array. The second disk is located in the D630's media bay where the CDROM used to sit. This setup works perfectly.
BUT: When both disks are in place, the system goes into GRUB rescue on boot and says "out of disk".
As soon as I remove the HDD in the media bay, it boots correctly. I have not tried removing the internal HDD. I can push the media bay HDD back into the system right away before boot continues and then I don't even need to rebuild the RAID.
Of course, I installed GRUB on both disks.
I checked and re-checked the cfg files, but everyting seems fine.
md1 : active raid1 sdb5 sda5 11881408 blocks [2/2] [UU]
md0 : active raid1 sdb1 sda1 964878272 blocks [2/2] [UU]
00:1f.1 IDE interface: Intel Corporation 82801HM/HEM (ICH8M/ICH8M-E) IDE Controller (rev 02)
00:1f.2 SATA controller: Intel Corporation 82801HM/HEM (ICH8M/ICH8M-E) SATA Controller [AHCI mode] (rev 02)