The easiest way (but possibly much slower) is to make sure the computer and My Clouds are all connected using Gigabit networking (no WiFi) and connected to the same Gigabit router or switch, then use either Mac Finder or Windows File Explorer to drag the files from one My Cloud to another My Cloud. Mapping the My Cloud may make things a bit easier to copy. Start the copy and let it run overnight.
The faster way is to copy direct from one My Cloud to another using SSH. The problem is it takes some knowledge of how to setup SSH and issue SSH terminal commands. Unfortunately there is no easy way to explain it in simple terms. For example the following discussion talks about how to use SSH and Rsync to copy files from one My Cloud to another.
