2nd HDD in Early 2011 MacBook Pro ejects during large file transfers, but only in OSX
The answer:

I took the 1TB HGST Travelstar 7K1000 and gave it a firmware downgrade.  HGST emailed me application to set the port to either, Sata I, II, or III.  Reducing it to Sata II solves the problem.

No more dropping.

Has somethgn to do with Apple/intels first venture into the Sata III world back then.  I used the less expensive Opti bay kit.  Not the more expensive Extreme they have created to solve the problem.

Works great and is plenty fast for a seconday drive.