I bought an 800x 16GB Lexar Professional yesterday (49€, dammit!), and the first time I inserted it I had the same result, but after reading it from the computer to see if the card was ok and inserting it again on the OT, it’s back to work again. I don’t know what happened or why, and I’m afraid to test the damaged card again, my confidence in this machine has been reduced to its minimum… 
I’ll give it another chance, but if this happens again I’m suppose I will go back to an MPC and do the Octatrack-ish work in Ableton.
As a result of this experience, the sketches that I had into the CF until last week have been lost (I tried to make an image backup of the card before formatting it, but I’m afraid that data was corrupted…). That was my “learning” work into the OT so nothing really critical, but learning with this machine it’s possible to achieve really great sounds and ideas and it’s a shame to lose them.
@miketheman yes, it was the 16GB Kingston 266x that came with the OT.
So guys, nothing new but: ALWAYS make backup…