>>Another thing to consider - if both source and destination are the same NAS, is there a "native" copy API you could or should be using instead of Windows file system calls?
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>This is something we would try to avoid. I am not aware of such utility.

By avoiding it you would avoid some extra work for yourself. But, it might eliminate the edge-case errors you're seeing. What's happening now to copy a file is pretty complicated:

Read from NAS native file system (maybe scanned by AV on NAS) -> NAS SMB emulator (e.g. Samba) -> network transfer to Windows machine performing the copy -> temp file on Windows machine (maybe scanned by AV) -> network transfer back to NAS SMB emulator -> write to NAS native filesystem (maybe scanned by AV on NAS)

A native call would eliminate a lot of that.

Performance might be a *lot* better. It might reduce pressure on both the NAS and the LAN, that might be an attractive feature you could sell to the client.

You told John there is a secondary process running - what is it?