Comment 9 for bug 947738

karl, if the install had *actually* happened to an ntfs partition, instead of to an ext4 partition, that would exactly explain the symptom we're seeing here (since ntfs doesn't support suid executables). If you still have this install available, can you mount it and check what the filesystem type is? Can you also show the output of 'ls -l /usr/bin/sudo' on the target partition, as requested above on the bug?

It's certainly possible there's a bug here that's preventing the partition from being wiped even though you marked it for wiping; or the issue could be something else entirely.