Can't add VM guest Exchange server to existing DAG

I've had a 2-mailbox DAG running for a couple of years, and now I'm getting ready to retire the hardware & move the servers to VMs.

MB1 & MB2 are running Exchange 2010 SP3 on Windows Server 2008 R2.
MB3, the one I'd like to add, is Exchange 2010 SP3 on Windows 2012 and is a virtual guest.

All three mail servers are visible in the console, and I can create mailbox databases on the new server, no problem.  However, I just can not get MB3 to join the DAG.  When I try to add it I get a vague error "Failed to initialize cluster with error 0x80004005."  

Is there any compatibility issue you can think of that would be causing this problem?  I've checked permissions, ran the cluster config check on the VM, confirmed the IP addresses are fine - just can't find what I'm missing!

Any thoughts would be sincerely appreciated.
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Do you have 2 nics (replication and network?)
Can you ping between them?
Any firewalls in the way?
One other thought, are the mailbox paths (drive letter and path) the same on all servers for the databases and logs?
You know this already but I figured I'd ask.  Usually that error is cannot find a path of some kind.
CHR3800Author Commented:
I've got one NIC on each of the nodes, including the new one. This has always worked in the past (between the two Windows 2008 R2 servers).  I can ping all servers from all servers.  There aren't any firewalls in the way, in fact these servers are all on the same subnet/switch.

The drives & paths are all the same, so we're good there.
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CHR3800Author Commented:
Paul Cunningham over at ExchangeServerPro says I can't mix OSs in the same DAG.  So... guess that's why this isn't working.

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Thanks for that!  I was under the impression Exchange version/SP/Rollup had to match.
CHR3800Author Commented:
No further work necessary on this question; it can be closed.
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