I would suggest to ask in the Isilon section what tool Isilon prefers
you didnt even mention if the data is NFS, CIFS or multi-protocol
Just to begin the discussion these nearly free tools are very useful in all NAS data copy situations:
CIFS/SMB: robocopy, EM Copy (EMC exclusive)
General (Windows/Linux/Mac): Beyond Compare 4 from Scooter Software ($50/license)
More information about the complexity of your environment would help generate better recommendations.