Have you tried determining what's in mdadm.conf in the initrd file? It might be getting some incorrect assembly instructions for md0. 

On 22 Dec 2010, at 1203, Pann McCuaig wrote:

> Greetings SL fans,
> 
> Sorry for the length of this post, but I'm hoping someone can come to my
> rescue and want to provide sufficient context.
> 
> Recently I had a hard drive failure on a Sun X4600 box running SL4.8.
> The box has four drives; the drive that failed was /boot (only). The
> other three drives make up /dev/md0.
> 
> The /boot drive was not backed up (headsmack!).
> 
> I have created a rescue USB stick based on the System Rescue CD, which
> boots via grub. I can boot the System Rescue CD successfully, and it
> sees /dev/md0, which I can then mount and read from and write to.
> 
> I have attempted to create a kernel and initrd image to add to the USB
> stick that will boot the box as if the kernel and initrd image were on
> the failed (and now removed) boot drive.
> 
> I built the kernel and initrd image on a box similar to the box with the
> failed hard drive. I replaced /etc/modprobe.conf with the file from the
> target server, and then did 'yum install kernel-largesmp'. I copied all
> the resulting kernel-related files from /boot to the USB stick, as well
> as the appropriate directory from /lib/modules.
> 
> I then restored the helper box to its original state.
> 
> I booted the System Rescue CD on the target system and copied the
> /lib/modules directory into place on /dev/md0. I fixed up grub/menu.lst
> to have a stanza to boot the newly created SL4 kernel. I rebooted the
> box, and everything seemed to be going swimmingly, until . . .
> 
> . . . the booted kernel seems unable to build /dev/md0 and the boot
> process fails.
> 
> In the original configuration, the boot drive was /dev/sda, and the
> drives making up the soft RAID partition were /dev/sdb, dev/sdc, and
> /dev/sdd.
> 
> The System Rescue CD detects the USB stick as /dev/sda and the three SAS
> drives as sdb, sdc, and sdd. All is well.
> 
> It's not clear to me what is going awry with the SL kernel, but as
> the boot verbiage scrolls by, I see /dev/sdc referenced twice, and no
> reference to /dev/sdd. When the kernel attempts to assemble /dev/md0, it
> uses /dev/sda, /dev/sdb, and /dev/sdc and this fails and /dev/md0 cannot
> be mounted and the kernel panics.
> 
> Help, please. Suggestions? Thanks.
> 
> BTW, I've put both SL4.8 Disc One, and the SL4.8 Live CD on a bootable
> USB stick; both boot successfully, but I was unable to find a way to
> make either one recognize /dev/md0, much less "rescue" me.
> 
> Cheers,
> Pann
> -- 
> Pann McCuaig <[log in to unmask]>                212-854-8689
> Systems Coordinator, Economics Department, Columbia University
> Department Computing Resources:
>               http://www.columbia.edu/cu/economics/computing/