How to add one more DC to the existing 2008 domain/ forest?

Our only one existing dc is on a 2008 server. I just set up a new 2008 R2 and wanted to add it to this domain using dcpromo.exe. But I got an error saying I have to run adprep.exe /forestprep. Okay, it thought I just went ahead run it on the 2008 R2 but I got another error saying I have to run adprep.exe on a DC.  Okay, then I have to run adprep.exe on the 2008 server as it is only one dc at this moment. But question to you, which version of adprep.exe should I use? Should I use the one from 2008 R2 or from the existing 2008 server's ?  Please help.
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Cliff GaliherCommented:
Run it from the 2008 R2 disk. You not only need adprep, but the schema definition files. All of those are in the same folder as adprep on the disk.

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R2 uses a different schema.... it will update the forest and such before adding your second DC

once you go to R2 you cant go back....
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