After firmware update on HP ProLiant you may encounter following trouble: the machine failes to boot form HDD or USB stick.
The system check routine displays valid boot sector on HDD, but the boot still fails.
The possible reason: check if your box has an external USB HDD attached. Try to detach the USB cable, restart the machine - and enjoy!
1. Boot the box and enter the boot setup
2. Insert floppy boot option between HDD and USB (you shouldn't have any floppy drives, but the boot option is still here)
3. Reboot the box