I performed a full factory reset on My Cloud (V04.05.00-327) firmware on a MacBook high sierra 10.13.6. I also have a My Book Essential USB plugged into the back of the My Cloud for additional storage. I assume I can access that drive separately and store stuff to it since I can see it in the My Cloud app share directory. There are three Folders on that drive that contain photos. I can access them all. However, when I try to create another folder I get “Error: Directory creation failed”. I that because it thinks it is still tied to the previous version of the My Cloud before I did the factory reset? Do I need to blow it away and let it find the new restore My Cloud?
I would log in with ssh, and check dmesg output. It is possible that the volume was not cleanly unmounted in the past (has the “dirty” flag set), and so the MyCloud is mounting it Read Only.
The fix for that, would be to down the mycloud gracefully (reboot option, then pull power when it is still early in the boot process), attach the external drive to a computer, and then check the drive for errors. If no errors are found, the OS should reset the dirty bit, and then the mycloud will mount properly when you re-attach.
That’s where I would start anyway.