Do 12.5mm drives fit into Precision M6400?

Also, do I need to buy a caddy or something to put in a 2nd HDD into the M6400 or is it already present in the laptop?

 

From my reading some people got it others didn't, most likely though you probably won't have it. You can always check, it isn't too hard IIRC.

You will need both the Hard Drive caddy and the Interposer. Both will run you anywhere from $10-$30~. You can try talking to Dell and see if they can provide you or sell you the part at a discount.

 

The M6400 should already have everything you need to add a 2nd HDD in the system.
The M6400 uses a 9.5MM optical drive, not a 12.5mm.