LED blinks at a slow breathing rhythm after a disk replacement

Dear all,
I replaced both of the HDDs in my WD My Cloud Home DUO after the original HDDs were full. Later I found that one of the new drives was a failed one, so I put both of the original HDDs back to the My Cloud Home DUO device. Then I found that the LED on the device keeps blinking at a slow breathing rhythm for days, and in my WD discovery program the device was offline. I do not want to reset my device, and is there anyone who knows how to fix this issue? Or is this issue normal? Very appreciate your inputs.