So recently my storage space came up with this reduced resiliency issue. However, as in many other cases around the net, all of the drives showed up OK, making it very hard to identify the failed drive.
As suggested in other posts, I unplugged the Toshiba and plugged it into my desktop. Then the WD that was plugged into my server became inaccessible.
Since I have solved the problem by myself, it is obvious that I am not asking for help here. However, I think this problem is serious, because if we are dealing with a corporate environment and multiple drives, they will not be able to find out the failing hard drive one by one like us casual users can. I really like Storage Spaces and ReFS, so I hope MS engineers can fix this problem.
Thanks.
P.S. When I was removing the failed drive from the pool, a "failed to remove drive" error showed up, although the drive was removed successfully. Thus I think the Storage Spaces system needs more polishing as well. Windows Server 2016 Technical Preview 5.