Comment 2 for bug 1485708

What exactly do you mean when you say "emergency mode"?

The expected behavior, when you boot Ubuntu and have a filesystem referenced in /etc/fstab for which the device is absent, is that the system will prompt, via plymouth, whether to continue waiting for the filesystem or to skip mounting it. This should be the case up through Ubuntu 14.10. For Ubuntu 15.04, with the switch to systemd the behavior will be different; but should still not result in the system booting to anything that could be considered "emergency mode".