I assume the WD 1TB is an external USB 3. hard drive? If so is it connected to the My Cloud or to your computer? There are known speed issues for some when copying from certain USB devices attached to the My Cloud’s USB port. The generally recommended method for copying large amounts of data from a USB drive to the My Cloud is to connect the USB drive to the computer, then connect the computer to the same router/gateway/hub/switch via Ethernet cable as the My Cloud is connected too. Copying over WiFi will often be slow. If possible use Gigabit router/cables.
Have you read through…
https://community.wd.com/t/before-you-pack-up-your-wd-and-return-it-lets-talk-about-copying-speeds/91887
