Can I set up WDMyCloud as an rsync server for a Synology NAS?

You don’t need anything but ssh to the box. After you enable ssh and set the password, then you can do ssh sshd@mycloudex2ultra.local (that’s right for mine). Once you know that works, you can have rsync ssh to the server and start the rsync server there. Not quite as fast as setting up an rsync daemon, but it is more persistent, and less prone to error.

One thing you have to be careful about is that if you have setup a user “dan” (as I have), and then you rsync to sshd@mycloudex2ultra.local:/dan, that will be wrong. You have to do it to the shared directory where there is space.

I use the df -kh command to check this. But also look around. On my box, there is a directory /shares/dan which is a symbolic link into /mnt/HD/HD_a2/dan/ . So, I’m free to use either path, or any other path under /mnt/HD/HD_a2/.