External Drive Unexpectedly Ejects while transferring files

I've got a mid 2012, 13-inch MBP. I replaced the optical drive with a 1Tb HGST Travelstar. I used an MCE Technologies Optibay kit. I wanted to carry an on board mirror of the boot drive.

The computer treats the new drive as a mounted, external drive. There are numerous reports of external drives being unexpectedly ejected during large file transfers. Does not happen for small moves, say under 4GB at a time.

This essentially eliminates the possibility of using the drive as a backup. Purportedly, this started to occur after the 10.8.5 Mountain Lion Supplemental update.

Any thoughts on a remedy?

Thanks,

J