An interesting phenomena. I have two first gen 4tb/bay My Cloud Mirror devices purchased at the same time. Literally days from each other both drives acquired the same status (Drive 1 & Drive 2 went RED).
I have been working on both of them on and off for several weeks. Of course the first thing I did was removed the drives, purchased two new 4tb red drives, purchased two low end 4 tb drives, retrieved as much data as possible onto the cheap drives using EaseUS Data Recovery (note: WD Wondershare Data Recovery performed slow and lack ability to fully recover content so I switched with better success), updated firmware on devices, and then worked on getting devices back up and running.
First device I was able to simply re-initiate the RAID1 and the device formatted the drives so I am up and running.
Second device continues to not want to recognize bay 2. If I remove the disk drive from bay 2 and boot, bay 1 drive is readable but of course no RAID1. If however, I boot with drive in bay 1 and bay 2 I get a message intimating there are no drives inserted. This is the case even if I swap from original red drive to a newly purchased red drive.
No curiously, if I insert a red drive in bay 1 and a cheap 4tb drive in bay 2 I am off to the races with all systems go.
I am not making any conclusions but evidence is two gen 1 mirror devices fail almost simultaneously and newly purchased WD red drives exacerbate the issue. hmmm.
Perhaps solution is to move away from WD. Not certain yet. Still working the issue.