When you say; “The MC is hooked up to Win 10 computer via ethernet cable” does that mean you have the My Cloud directly connected to the computer without having it connected to a router?
Do you have the same problem copying files to other user Shares created through the Dashboard?
Other ways to test and isolate the problem. Do you have another computer or mobile device you can test to see if its the Windows 10 computer or the My Cloud? Have you tried using the WD My Cloud Desktop for Windows software to copy a file (or files) to the My Cloud?
In the past with prior firmware versions people had problems with certain folders being public only access post firmware upgrade.
https://community.wd.com/t/folders-within-public-share-read-only-after-firmware-update/96613
