First, start by reading the following thread to understand the various issues that can impact copying speed.
Next, there are several ways to move data from one My Cloud to another. This assumes one is connecting both My Cloud’s to the same local network using Ethernet.
Use Rsync to copy the data from one NAS to another.
Use Safepoint (v3.x/v4.x firmware) or Backup (v2.x firmware).
Use a external USB drive in conjunction with WD My Cloud Desktop, My Cloud mobile apps to copy the files.
Use Windows File Explorer or Mac Finder to move the files.
There are other methods but those are the basic ways and each has its own issues. Using Rsync is probably the best and possibly the fastest copy method but requires knowledge of issuing command lines via the SSH command line terminal. Safepoint/Backup will allow you backup one My Cloud to another and from there move the files/folders/shares around on the second My Cloud as needed to rebuild the folder/share structure. Using a USB drive is another method but one would need to use the My Cloud Desktop or WD My Cloud mobile apps or use Rsync to copy the files from the My Cloud to the USB drive, then move the USB drive to the new My Cloud and reverse the copy process. Using Windows File Explorer or Mac Finder is probably the slowest method as the data would flow from one My Cloud, through the computer, then out to the new My Cloud.
Rsync or similar SSH command line copy is probably the best method. Here is one link to get one started using Rsync:
How to Migrate From an Old NAS to a New One Overnight with rsync
