Spare ethernet card prevents thin client from booting

[Dan Moore]

I have two ethernet cards in a machine that dual boots into Windows and a Hardy Heron thin client.  When I disconnect the cable from ethernet card that is plugged into a PCI slot, and boot LTS from the motherboard's NIC, the machine can't boot.  If I remove the PCI ethernet card, everything works.  

It is tedious to open the case just to boot into LTS.  Is there a way to get around this?