In addition to what Rac8006 said, the My Cloud motherboard is not programed for a specific sized hard drive. If you search this subforum (magnifying glass icon upper right) you will find more than a few users who have replaced the My Cloud hard drive with a larger (or smaller) hard drive.
The firmware that drives the My Cloud resides on the hard drive not the motherboard. Like Rac8006 indicated you must find out what the current My Cloud version is and use the same version, if possible. Otherwise you will have to use on of the various unbrick methods (again search the subforum) and wipe the drive (and its data) to install a different model’s firmware.
One can use Linux or a driver for Windows to access and backup any user data from the My Cloud hard drive.
