On running ADPrep passes all prerequisite, but fails in the last stage with the below error:
ADPrep execution failed --> System.ComponentModel.Win32Exception (0x80004005): A device attached to the system is not functioning.
ADPrep log says:
Adprep requires access to existing forest-wide information from the schema master in order to complete this operation.
I'm using the credentials of Enterprise Admin (schema admin too) couldn't find why it still throws access denied error.
I'd comment on the original question so I can gather more info before posting my answer, but I don't have enough reputation yet as I'm new; go figure right :)
Anyway, I've ran into this twice for two different clients, one that was running SBS 2003 and another running 2008 R2 like you are. Both times had a few different tweaks but ultimately the solution turned out to be this:
Remove the Active Directory Domain Services role from the 2012R2 server if you have already installed it.
I know you already said you had Enterprise and Schema rights, but make sure you're using an account with Enterprise, Schema, and Domain admin creds.
Reboot the 2008 R2 DCs; yes all of them.
Go to the 2012 R2 server and add the Active Directory Domain Services role and reboot after it successfully adds.
Once logged back into the 2012 R2 server open Server Manager and on your post installation tasks you should see something similar to this screenshot:
click on the link "Promote this server to a domain controller" (ADPrep (both forest and domain) will run automatically without manually needing to have you run it from a command prompt)
If you are still having trouble please look for the value in ADSI Edit for your server entitled "ClaimIsValueSpaceRestricted"; it will either be true or false. Let me know if it's true or false and if it's greyed out or not. We can troubleshoot further should we need to, but steps 1-7 should resolve your issue.
Here is what I did to get it to work
I had the exact same error and the solution from https://community.spiceworks.com/topic/304023-adding-server-2012-as-dc-to-server-2003-domain-getting-adprep-error?page=2 solved it for me:
Check the Remote Registry service on the 2003 domain controller is configured as follows:
Startup type: Automatic
Service Status: Started
Security context: NT Authority\LocalService (In Log On tab of remote registry service)
Then promote the 2012 server again.
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