If you do a site search (via the magnifying glass icon in upper right) you will find quite a few threads on using rsync to sync data between a My Cloud and other devices. Here is a couple of those links.
https://community.wd.com/t/can-i-set-up-wdmycloud-as-an-rsync-server-for-a-synology-nas/93759
https://community.wd.com/t/rsync-backup/96208
https://community.wd.com/t/success-3-rsync-backup-between-my-cloud-devices/93945
