I have had this issue when running drives mirrored with storage spaces, and then later separated them.
What happens is that both drives has the same drive ID, open disk manager and you can se something like that.
What exactly i did to fix it im unsure of, but windows got confused about the drive states when they appeared identical in disk manager.
So this is 100% a software issue. Make sure you delete both drives from spaces, and check disk manager if they are mirrored there.