This is very strange. The two motherbords (GA-Z77X-UD3H and GA-Z77-D3H) are practically identical from the chipset point of view - http://www.gigabyte.com.ro/products/comparison/list.aspx?ck=2&pids=4441,4327
Despite this, yours installs the Esxi 5 with the Atheron driver without a problem, but mine doesn't seem to find the network board.
Do you still have the custom iso for the esxi 5.1 used in the installation?