With VSAN6.1, Virtual SAN monitors solid state drive and magnetic disk drive health and proactively isolates unhealthy devices by unmounting them. This has happen to my home lab and wish to share how I recover the unmounted disks.
You can read more about this feature on http://cormachogan.com/2015/09/22/vsan-6-1-new-feature-problematic-disk-handling/
As you can see, the disks in one of my hosts are unmounted. I have tried to find a menu so I can remount the disks but could not find one. Put the hosts into maintenance mode and restart the hosts also did not recover the unmounted disks.
I have to erase the partitions (both HDD and Flash) for that host, claim the disks again before I could observe the disks are mounted correctly again.