I used Robocopy to initially populate my ReadyNAS NV from a windows XP workstation. The total copy size was around 30 GB and I was very happy at the copy performance over my 100MB lan (much faster than the Drag-n-drop method).
I was happy until I repeated the identical robocopy command only a few minutes later... I noticed that a bunch of files got copied again. Since these had already been copied over once, they should not have been copied the second time.
I finally found that using the FFT option solved my problems. Using the FFT option tells Robocopy to assume FAT File Times (2-second granularity). After adding this option, I had no files recognized as 'newer' or 'older' that required re-synching on the ReadyNAS.
So now I always use this:
Robocopy \\source \\destination /e /z /fft
Links for Robocopy:
Snippet from http://www.screamingjungle.com/mt2/files/robocopy.pdf
Some third-party operating systems declare 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.
Hope this helps ease someone else's frustration in the future...