How to best transfer 30GB to WD My Cloud - when it breaks midway through!

My Cloud OS 3 Personal & Network Attached Storage My Cloud
2

just use whatever app you are familiar with. Either access the shares directly or map them.
The less dependent you are on proprietary apps the better.

Also look for “robocopy” command for Windows.

Added: Read this What is the WD Quick View app doing? - #2 by cat0w