I've made quite a few changes recently and have been up against this issue for a while - I have two network adapters and can't point the right one at eth0.
Onboard is gigabit, and I have a 10gb fibre card that I want to be the default. I want to have the onboard activated so that I can use wake on lan which doesn't work with the fibre.
I've managed to lose the option to assign eth0 and eth1, and can't get it back. Have tried reverting to rc7 and back, and many many reboots but it fails to show. It's left me with the wrong adapter as default.
Diagnostics attached. Goodness knows what horrors lurk within....