Comment 2 for bug 665195

Sorry, but I think you've misunderstood the purpose of /etc/init.d/umountroot. This script is not supposed to *unmount* the root filesystem, it's supposed to remount it read-only. A plymouthd process running does not prevent this - and plymouthd is supposed to stay running up until the very end. Have you checked in your modified umountroot script what processes are holding the root filesystem open for *write* at this point? Those will be the processes preventing / from being remounted ro.