After cloning a system or replacing network card or for some other reasons, I have seen eth0 missing and ifconfig only showing eth1
removing the following file and rebooting resolved;
/etc/udev/rules.d/70*
Also this file can be edited and udevadm trigger refreshes.
This may also apply to /dev/hda /dev/hdb /dev/sda /dev/sdb etc (to be verified)
