We had a system with a mirrored boot disk. One of the disks failed. And we were unable to boot from the other, because the boot device in OBP (~BIOS) pointed to a device-specific partitIon. When we manually booted from the live device, it was lacking the boot sector code, and wouldn’t boot. When we booted from CDROM, the partitions wouldn’t mount because the virtual device mapping pointed to the dead drive.
This was a gas futures trading system, and rebuild wasn’t an option. Restoring from backup woyld have lost four hours of trades, which would be an extreme last resort.
A coworker and I spent all night on the box. We had a whiteboard covered with every stage of the boot sequence broken down, and every redirection we needed to (a) boot and (b) repair the system. The issue started mid-afternoon, and we finally got it back up by around 6:30 am.