Could you try using HobyCopy instead? It is a SourceForge project. Its description from there:
HoboCopy is a backup/copy tool. It is inspired by robocopy in both
name and in functionality. It uses the Volume Shadow Service (VSS) to
"snapshot" the disk before copying. It then copies from the snapshot
rather than the "live" disk.
This should prevent any issues with locked / in-use files. This does assume that VSS is enabled ("Previous Versions") for the volume you're trying to copy from.
Some additional details about VSS are available at http://en.wikipedia.org/wiki/Shadow_Copy.
You can also find a number of tools that can be used to script VSS operations, including mounting to a drive letter / NTFS path, such that normal non-VSS-aware tools e.g. XCopy can still be used.
Alternatively, if you open the properties for a VSS-enabled drive, you should be able to use the "Open" option from the "Previous Versions" tab, and see a URL like
\\localhost\C$\@GMT-2011.12.29-04.10.15. This URL may also possibly be useable by XCopy and other non-vSS-aware tools.