I had the same issue and did the same as you. I could not figure out how to correct the issue. You could try
This may force it through a check & upgrade.
Howvever I ended up reinstalling and when I got the error again I did the following.
pacman -Sf filesystem
rm /etc/mtab /etc/profile.d/locale.sh
There is a glitch in the Kernel prior to the current version 3.2.8-1 which is on the install iso that is causing it. The above code did the trick and I was able to continue my system build.