Fragmentation? seems unlikely but possible that you dont have a continuous block of storage available given that you have huge amounts of free space.
Might be a memory issue on the receiving server? Try unbuffered robocopy (/J)
Since the receiving server is running windows - check the event logs to find what the backend error was.
Here is an older KB, but should be still mostly relevant - in the section labeled "cluster size too large", you'll find an explaination of the blocksize issue people are talking about. You seem to have wayyyy too much free space for this to be an issue. Essentially the failure mode is that blocksize is the minimum size on disk, and if you have something like a 64k blocksize and a million 1KB files you would have 62.5GB of disk used to store only 0.9GB of data. You would need an incredibly high blocksize and an incredible amount of small files to fill in a 50TB array though.....