How to properly migrate file server data?
The version of robocopy that is included with Server 2008 does not include the security attributes when you use the /MIR option. You have one of two options:
Install the version from the Server 2003 Resource Kit (http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=17657) and command you already have.
Run the command you already have and immediately run the following to copy the security attributes onto the files. Robocopy \\oldserver\Data E:\DATA /COPY:S
The parameter /sec needs to be added to your command-line. This will cause the /mir to copy security attributes. However, you really, really want to do that to a fresh target. Because of how robocopy works, it'll apply security settings to a directory and then proceed to copy in the child contents. This way those children get their inherited rights correctly.
Do /sec after already doing a copy, and you'll end up waiting for the rights-apply to percolate down the tree for each directory (or file) with explicit rights set.
I used the /mir /sec combination frequently to move some rather large directories. It works great.