I have a My Cloud Home Duo 12 Tb.
I can access it through the “my cloud” web interface and upload files there.
I can also access it by Windows Explorer (I have mounted the network partition there).
However, I can not see the files I uploaded by the web interface in Windows Explorer. Also, if I upload a file in Windows Explorer, I can not see it in the web interface.
I have also mounted the network partition using the credentials from the web interface (email and password) but the result was the same.
This has been asked and answered a few times before if you check this subforum. In most cases, this occurred after mass migration and it takes some time for the MCH to index the files and repopulate the database required for display by the webapp.
You will see more and more files reappearing after a while but it could take hours to days depending on how much data has been dumped onto the MCH. The MCH will become slower as any disk drive would, as it approaches the limit of its capacity.
The MCH is a slow device with hardware that is more than 6 years old, it is cheap storage and users should not expect high performance from this personal cloud storage for the home.