It happened with my Thunderbolt Duo on Macbook Pro 10.11.6. Randomly ejected and then not mounted at all. Very frustrating.
Here’s my Post:
I solved it now, Restored my MBPr from Time Machine a couple of days back solved the problem. I suspect it caused by the SysUpdates from Mac Store.
Did you update anything from Mac Store recently?
The best way to check was to use other Mac if available. or a PC using Paragon HFS+ installed
Cheers
