Comment 26 for bug 251164

Commenting here since bug #324997 has been marked as duplicate and the behaviour appears to have changed since the original report. With Ubuntu 12.04, booting with a degraded raid (due to missing disk) causes the boot process to stop at "Begin: Waiting for encrypted source device... ...". There's a timeout of around 180 seconds after which a busybox prompt becomes available. It's possible to continue from that prompt by commanding "mdadm --run /dev/mdX" followed with ctrl-d. That causes cryptsetup to find the device, prompt for a passphrase and continue the boot procedure.

I noticed that this problem occurs only when a disk is missing from the raid. Setting one disk as failed and then booting with all disks but still in degraded state will result in some warnings during the boot process but there will be no waiting for timeouts or busybox prompts.

Another suggestion I found in several places and ended up testing was to replace the UUID in /etc/crypttab with the device itself (/dev/mdX). That results in no timeout and cryptsetup asking for the passphrase but it will not accept the correct passphrase as long as any disk from the raid is missing. There's no busybox prompt available either resulting in a system that can't boot.