I am having this issue as well. I'm not sure where the problem is starting. I have removed two drives from my pool to keep them from "falling" out of the pool. I have two others that just started this same behavior. I thought it might be a compatibility issue with my drives and the storage controller but I have not seen the drives disappear altogether from device manager or disk manager in windows. I'm at a loss and really would love to hear from any solutions other users have had to this problem.
edit: I wanted to add that I have not resized any partitions. no software has been added. no configuration changes have been made before the issue occurred.