In my last job I ran into a scenario where a hard drive failed while a system was still running. When I attempted to reboot with and /sbin/reboot it failed with a rather cryptic message.

Honestly I had never ran into this issue before. So I turned to google and found this excellent Linux Journal Article. Explaining a excellent way to reboot a system without the reboot. It involves doing the following:

echo 1 > /proc/sys/kernel/sysrq echo b > /proc/sysrq-trigger

The system will immediatly reboot (without going through the normal unmounts so there will definitely be a need to fsck your disks when coming back).

Hope it helps!