Replace optical drive with hard drive in mid 2012 non-retina MacBook Pro
I have heard of certain WD drives having hardware aggressive power saving features where the drive will shut down after a rather short period of inactivity. Worse, it doesn't tell the OS that it's doing so, so you get into a 'wait race' where the drive is waiting to be told to spin up, and the controller is waiting for the drive to signal 'ready'.

This usually isn't a problem if the computer shuts down the drive to save power:  It then knows that the drive isn't ready, and sends the appropriate startup.

Try asking with more detail:  Drive, kit, etc.