If you're setting up an ec2 instance in the Amazon cloud and are setting devices (EBS volumes) to mount on reboot by adding them to /etc/fstab, make sure you put "nobootwait" in the options, like this:
/dev/xvdh /export/home ext3 auto,rw,nobootwait 0 0
If you don't, then if the drive isn't present, the boot process will hang, either with a blank console output, or with:
The disk drive for xxx is not ready yet or not present
Continue to wait; or Press S to skip mounting or M for manual recovery
and as the keyboard is miles away, you can't press that key.