Have you tried initializing the transfer from the "whs" instead of from Windows 8 to the WHS? Try that, go into the WHS and try to move it from Windows 8 to there, from within the WHS.
The only thing I can think of, is that at the very end of that transfer, it may be pulling up a Permission dialogue on the WHS which you can't see on Windows 8. Because it is possibly moving the transferred file from a Temp folder on the WHS to the folder you chose as the destination.
You may also try checking the permissions of that file, and give Administrators full permission to handle it.