FYI there is a dedicated subforum for the PR Series. This subforum (My Cloud) generally discusses the single bay/single drive My Cloud units.
https://community.wd.com/c/network-attached-storage/wd-pro-series
The PR Series has an embedded backup feature in the My Cloud Dashboard. It allows one to backup the My Cloud to either a USB drive attached to the My Cloud or to another location. See Chapter 8 Backing Up and Retrieving Files in the PR Series User Manual (http://products.wdc.com/library/UM/ENG/4779-705142.pdf#page=50).
One can also use Rsync to backup files to USB hard drive attached to the My Cloud or to another network location. I use a variation of the methods discussed in this thread: https://community.wd.com/t/alternative-to-safepoint-reposted/140685
For backing up a My Cloud to a USB drive attached to my computer(s), while one can use a dedicated backup program, I ended up using Free File Sync (https://freefilesync.org/) and mirroring the files to a USB drive attached to another computer on the local network. That program is free and pretty simple to setup and configure.
