The same thing suddenly happened to me yesterday with two of my three MyCloud Duo Thunderbolt drives that are connected to my MacBook Pro, but only immediately after yesterday’s El Capitan latest beta release was installed. All three drives worked fine on earlier El Capitan beta releases. The two that now no longer “work” mount for a couple of seconds on booting, but immediately dismount, while the other drive still works fine, as before.
I have since tried the two MyCloud Duo Thunderbolt drives that are unmounting on my El Capitan OSX MacBook Pro on a Yosemite OSX MacBook Pro, and they are working fine.
