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failed to add device microsoft synthetic ethernet port

We have had to replace our motherboard due to a faulty DIMM slot.
The 2 VM's we have in our setup (Physical server running Server 2008 R2, VM SBS2011 and VM Server 2008 R2/SQL Server) were networked to the onboard NIC of that motherboard.
I have recreated a new Virtual Network Card for the Hyper-V to use (I have actually tried multiple onboard NIC ports). This new connection is accepted by the Hyper-V Virtual Network Manager, but when I go to the VM and try to install the "New" Network Card in its setting page, I receive the following "Failed to add device "Microsoft Synthetic Ethernet Port".  'SBS2011' failed to add device "Microsoft Synthetic Ethernet Port".  (Virtual Machine ID 3E108A89-1269-45F2-B3AD-40262F238C3A).

'SBS2011' being the name of the VM I am trying to install the newtork card into. The same error occurs when trying to do the same thing on the second VM.

All references I can find about "Microsoft Synthetic Ethernet Port" want to point me to Forefront or AntiVirus software in general it seems. However, this setup - in production - was running just fine with Kaspersky Anti-Virus 8. So I don't see that being the cause of the problem here.

I need to get this resolved before Monday morning if at all possible. This is our sole server and without it - well . . .

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2 Solutions
If you are running Kaspersky 8, it has firewall & NDIS filter. After you replace mother board, new network connection's firewall profile may change to public. So check it, try again by disabling ndis filter in network connection properties. Most of people say error is gone after add to exclude, so I think it is better to try it also.
Another thing you can try to create new vm with network card to see whether it will success or not for trouble shooting purpose.
wallsjkAuthor Commented:
Did another reboot of the physical server, stopped the Kaspersy Real Time AV (The network was Private not Public - but had been previously in my repair attempts). Then tried to install the Network Cards in the VM settings page and it finally took. So it may have indeed been an issue with the Kaspersky AV. But I had already tried several other possible solutions so I can't say with certainty that that was "indeed" the problem. For the record - I did not try the NDIS Filter scenario nor the New VM scenario either.

But may have definitely been a factor.

Thanks  upalakshitha for your help!
wallsjkAuthor Commented:
I had made my adjustments prior to  upalakshitha's solution submission. I feel we both had the same thing in mind overall.
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