One work-around is to use the built-in web file viewer app in the dashboard to move/copy files. The USB drive should automatically be created as a share when plugged in, and you can use the app to move or copy files between shares, so it can be used to make a transfer.
What you’ll find is that the connection to the dashboard will probably time-out before your transfer is done (especially if it’s 2TB), but it will carry on in the background anyway (although from that point onwards there’s no way to actually monitor or cancel it that I know of). You can only move one folder at a time, but with a bit of preparation (moving all the folders you want to transfer into a single mother folder, transferring that and then rebuilding your folder tree on the target end afterwards) it does work.
I used that method to populate my 5TB elements drive which I have hooked up to my MCM, took about a day to do the full transfer (also about 2TB worth of files, similar to yours). It has the advantage of being a direct USB3 transfer, so is about as fast as you can get. Don’t try to do it via a computer (other than for remote connecting to the dashboard), as (for example) doing a drag and drop transfer using Windows Explorer with the MCM and USB drives as SMB or networked share drives will just results in all files going via the computer, which slows everything to a crawl (especially if wifi is involved).
You are right that USB<>NAS backup seems to be somewhat unreliable (to be diplomatic) on recent firmware. It’s a shame, as it did used to work, and is something that should definitely be looked into by WD tech.
Editted to add - also depending on how comfortable you are with Linux, have a look at this post which may give you another option to do this from the command line.
