df Says, "Cannot read table of mounted file systems."
There is probably something wrong with your /etc/mtab or /etc/fstab
files. If you have a reasonably new version of mount, /etc/mtab should
be emptied or deleted at boot time (in /etc/rc.local or /etc/rc.d/*),
using something like
$ rm -f /etc/mtab*
Some old Linux distributions have an entry for the root partition in
/etc/mtab made in /etc/rc* by using rdev. That is incorrect--the newer
versions of mount do this automatically.
Some old distributions also have a line in /etc/fstab that looks like:
/dev/sdb1 /root ext2 defaults
The entry for /root should read simply /.