can't copy files to readynas with new 6TB drive

I have now tested the drive and it seems to be fine. I have also connected the readynas directly to the windows machine, the first two attempts at copying a 6.49GB folder failed. I then connected the readynas the the pc using a cat6 cable, the copy then instantly failed, I think this might be due to the unit needing to be restarted? Once I restarted the readynas the folder finally copied over with no error, which is good news. I then went for a larger folder (around 60GB), and this copied over fine as well.

I then switched the connection back to cat5e to see if the copy error would come back, I think it did once trying to copy over the 60GB folder, but did not when trying to copy smaller folders.

Once again, I went back to cat6 and didn't get any problems at all, this was copying various folders of up to 100GB in size.

Now I'm back on cat5e to see if the errors come back, and they have not yet. I've now copied 380GB over, averaging 35MB/s. I'm currently attempting to copy a 125GB folder which will take about an hour.

So what does this mean?