Root filesystem check fails after power failure during installation

During the "install" phase of the upgrade there was a power failure. After when starting up again the following errors are reported:

init: udevtrigger main process (420) terminated with status 1
init: udevtrigger post-stop process (428) terminated with status 1
init: udevmonitor main process (419) killed by TERM signal
The disk drive for / is not ready yet or not present
Continue to wait; or press S to skip mounting or M for manual recovery

Pressing M gives me the following message:

Root filesystem check failed.
A maintenance shell will now be started.
CONTROL-D will terminate this shell and reboot the system.

I had the same situation happen -- power failure during upgrade.

Get to the maintenance shell, and try this:

sudo mount -o remount,rw /
sudo  dpkg --configure -a
sudo mount -o remount,ro /
sudo sync
sudo reboot

(the remount/sync at the end is probably just paranoia, but just in case...)

When you get to this root shell, type fdisk -l to get a list of the drives in the system, then do a fsck on the / (using it's name from the list, for example /dev/sd5 partition.

To force a disk check, use sudo fsck -f.