So HP engineering said that this should work but they had not tested it. Make sure you are fully up to date on the firmware and the P410 should be able to go to the P400 safely. But.... caveat emptor, of course.

It was the motherboard. Tested the PSUs this morning with replacements and no issues with the originals. Swapped out the motherboard and the baby fired right up. We are currently recovering everything and anticipate zero data loss. No need to even fall back to the backups.

This should help.

http://www.techotopia.com/index.php/Creating_and_Managing_Windows_Server_2008_Mirrored_(RAID_1)_Volumes

@scottalanmiller said:

Was doing some research on this recently and found the HP documentation on how to do this:

HP Proliant Servers - How to Reset the HP ILO Inband Root/Administrator Password in Linux

It is very useful (if you have Linux running on the box in some form, or vSphere which is similar enough.) However, it turns out that some ILO firmware editions don't work with this and cannot be reset without being patched first. Important to know.

Thanks for the heads up.