Once you have the flashing red front LED after performing an unbrick, try accessing the My Cloud Dashboard with a web browser. Assuming you have connected the My Cloud to a local network router. Then perform a System Only factory restore via the Dashboard. Then you can try updating the firmware to the very latest v4.x firmware.
If you cannot access the My Cloud Dashboard after unbrick, try performing a 40 second reset via the reset button on the back of the My Cloud.
Yes a WD elements drive that’s been shucked will work. Experimented with an 12TB WD Elements drive in a first gen My Cloud a while back before moving to a Synology NAS.
