Because your using a computer to initiate the copying it is possible the data from one My Cloud is flowing through the computer to the second My Cloud. That would explain why your getting horrible copying speed especially since you indicated one of the computers you used was on WiFi.
Try using Rsync from the SSH command line. If you don’t know how to use SSH or Rsync do an internet search, plenty of tutorials out there. Here is one that covers using Rsync to copy to a variety of locations.
http://www.tecmint.com/rsync-local-remote-file-synchronization-commands/
Also use the forum search function to search for “rsync” as there are several past threads discussing how to use Rsync to copy data from one My Cloud to another location (or My Cloud). Here is one such thread.
https://community.wd.com/t/success-3-rsync-backup-between-my-cloud-devices/93945
Don’t forget to enable SSH via the Dashboard > Settings > Network if it isn’t enabled already.
