sudo passwd root -- operation not permitted

[joe at actionline.com]

Just installed Kubuntu 12.10 again (now on my 4th installation attempt
today) and still can't get it to work.

This time, I used the "manual" install procedure to install only the root
(/) partition since I already have /home installed on /dev/sda6 with all
my files.

But now I cannot access that /home partition.  I'm stuck in a tmp/guest
directory and I can neither change user to /home/joe nor can I use: 'sudo
passwd root' because all attempts say "Operation not permitted"

$ df
Filesystem     1K-blocks     Used Available Use% Mounted on
/dev/sda1       12396968  3571428   8195796  31% /
udev              469488        4    469484   1% /dev
tmpfs             192240      784    191456   1% /run
none                5120        0      5120   0% /run/lock
none              480592       84    480508   1% /run/shm
none              102400       12    102388   1% /run/user
/dev/sda6      175869792 23265668 152604124  14% /home
none              480592     9512    471080   2% /tmp/guest-3sbRVN

I am able to view /home/joe to confirm that all my files are there by
going to CTRL+ALT+F1 and logging in as 'joe' ... and in the F1 terminal I
can also use 'sudu passwd root' to reset the root password.  But then when
I go back to the KDE graphical desktop (F7) and open a terminal, I still
cannot get into the /home directory or su to root.

$ cat fstab shows this:

# / was on /dev/sda1 during installation
UUID=962718a7-f399-4173-8260-1d5c0249a8d3 /  ext4  errors=remount-ro 0   1
# /home was on /dev/sda6 during installation
UUID=5eb2a784-c640-4962-8a81-6f81a0ccca85 /home  ext4    defaults 0     2
# swap was on /dev/sda5 during installation
UUID=c0c7f1be-ff0f-4f18-a86d-a98bd91d0e62 none   swap    sw    0     0