If you have accessed the public share without providing your MyCloud credentials, windows will not let you access a private share with the MyCloud credentials. Windows does not allow mixed credentials for a network device.
If this is your problem, the error message will have an explanation.
Unmap your public drive, and remap all drives, forcing the use of your MyCloud credentials. You may have to reboot the pc after unmapping, or even use Windows credentials manager to delete the credentials association.
