Robocopy is very powerful utility with a lot of cli switch. Especially /MIR.
/MIR option activates mirror mode (i.e. deleting files in the destination that are not present in the source).
In some case may be necessary to use /FFT (FAT File Times) switch combined to /MIR.
Some third-party operating systems say that their volumes are NTFS, but only implement FAT-style file times with a 2 second granularity. When copying to such a destination from a true NTFS volume, file time rounding may occur, along with unnecessary copying of file data in subsequent jobs. If you encounter this situation, you can use the /FFT (FAT File Times) switch to tell Robocopy to consider file times to be identical if they are within 2 seconds of each other, even though both source and destination appear to be NTFS. This eliminates unnecessary copying in this scenario.
Snippet from http://www.readynas.com/forum
