what happens if you try to transfer on the local network without the software? Try to do this and see if you get the same result. Just to be clear i would also like to know if this is locally or remotely connected to the unit.
I have the same problem with the My Cloud Desktop for Mac. Using Finder is not possible with our macbooks since the uploadspeed is around 1MB/s and in 95% of the cases it doesn’t copy but gives an error after completing.