Have your read the My Cloud User Manual? (http://www.wdc.com/wdproducts/library/?id=439&type=25) In particular the “I need to migrate a large amount of data (50 GB or more) from a USB external drive to my My Cloud device.” in Appendix C.
The recommended method of moving large amounts of data from a USB drive to the My Cloud is to connect the USB drive to your computer, connect your computer to the same router, via Ethernet cable, that he My Cloud is connected too. Then use Windows File Explorer or Mac Finder to copy the data from the USB drive to the My Cloud.
The problem with connecting the USB drive direct to the My Cloud then using Windows File Explorer or Mac Finder to copy files from the USB drive to the My Cloud is the data may flow from the USB drive through he My Cloud, then through your computer, then back to the My Cloud hard drive. That generally results in very slow USB coping speed.
Do a subforum search for USB speed and you’ll find several threads discussing the low copying speed from the My Cloud USB port. There are some ways around it, from using the WD apps/Desktop program to copy or move the files to using Rsync or the Linux copy command via the SSH command line to copy files from the USB drive to the My Cloud…
Also see the following thread if you haven’t already read it:
