While attempting to backup to a NAS I am receiving a "The security identifier provided is not from an account domain" message. Upgraded from Vista which was working.

What model?Can you browse to the NAS through Windows explorer?

Thecus N3200 Pro. I am able to access the NAS through explorer and other programs.

Unfortunately I don't have that model but let me try with one that I have on Monday and let you know the result I get.You are using the native Win7 backup interface I presume?

Yes - using the native Windows backup interface. Interestingly enough files are copied to the NAS, but Windows still does not like something.

Is the backup still failing to the particular NAS device. If it is still and issue can you please post this thread on windows backup forum. Windows Backup team will followup further from there. ThanksAbhinav [MSFT]