Ok that is what I suspected from the start... your root (/) partition is NOT ext3 as you originally stated. It is formatted as reiserfs. Back in 5.0.x the disk statistics only gathered for ext3 partitions (they changed this in 6.0.x I think from looking at the script).
Anyway to get your / partition to show up in the disk statistics you just need to change what partitions it grabs data for. To do this you just need to change line 48 of /opt/zimbra/libexec/zmdisklog.
To get it to only show for reiserfs partitions (/ and /tmp, or device wise /dev/mapper/system-root and /dev/mapper/system-tmp) then change the line to read:
To get it to show both reiserfs and ext3 partitions change it to:
$MNTCMD = "mount -t reiserfs";
$MNTCMD = "mount -t ext3,reiserfs";