This page is part of the debugging series — pages with debugging details for a variety of Ubuntu packages.
When LVM/RAID/etc. don't work on boot
Boot with break=premount on the kernel command line, remove both quiet and splash for sanity.
(initramfs) rm scripts/init-premount/udev (initramfs) echo "" > /proc/sys/kernel/hotplug (initramfs) /sbin/udevd --verbose --suppress-syslog >/dev/udevd.log 2>&1 & (initramfs) /sbin/udevtrigger
Now press ^D and let the boot continue; once it has booted, please provide the /dev/udevd.log file.
NOTE: This does not appear to work on Lucid