Why does my hard disk (in MBP optical caddy) keep unmounting when writing? I replaced the optical drive in a 15" Macbook Pro (late 2011) with a new 2TB Samsung HDD in a caddy tray. It formatted fine and Disk Utility says it's OK.

The problem occurs when I try to copy a large set of files to the drive. It starts out alright, copies a few GB. Then it spontaneously unmounts in the middle of the copy and throws a Disk not ejected properly error.

If I open Disk Utility afterwards, it doesn't even see the drive. Restarting the computer doesn't bring it back. But if I shut down and then start up, it comes back on just fine.

Normally I would assume it's just a faulty drive, but the exotic nature of the optical drive caddy makes me suspicious.

Has anyone encountered this sort of problem? What is the likely cause? Any suggestions for troubleshooting prior to returning the drive? Thanks!
I found out that this particular model (late 2011 Macbook Pro) has an issue with the optical drive SATA: It can only operate reliably in 3Gb/s mode, but reports that it supports 6Gb.

The drive I was using supported both speeds, and is supposed to automatically switch to 3Gb when necessary. But because the bus reports that it's compatible with 6Gb, it stays in 6Gb mode. This works fine most of the time as the actual transfer speed doesn't exceed 3Gb. But when it does, the bus misbehaves and all sorts of problems occur.

After much frustration (and several different drives) I settled on putting a 3Gb (not 6/3) drive in that bay instead. Works great now. Incidentally, the other SATA drive bay supports 6Gb just fine.