SSD in Superdrive slot fails after 20 minutes

Hi all,

I added an SSD to my SuperDrive slot in a caddy and all seemed to be working fine, both drives showed up and all was good then about 20 minutes later i got a message saying that "the drive had been ejected incorrectly" or something like that. The drive has now disappeared from finder and Disk Utility. I took the SSD out of the superdrive caddy and put it in an external USB3 caddy, it's working fine. I even left this connected over night (all OK). This morning i put the SSD back into my Late 2011 MacBook and the same thing happened, 20ish minutes and then the eject message... Have i missed something?

Any help would be very much appreciated.

Thanks

Steve