Comment 1 for bug 462258

I did:
$ sudo dpkg-reconfigure mdadm

Was asked to boot in degraded mode (the default was 'no'-- will need to verify if I answered yes on install. update-initramfs was run and I rebooted without vdb. It booted in degraded mode. I shutdown and removed vda and added vdb back and booted and was presented with the initramfs telling me the disk could not be found.