[NAME] , the drive cages may have pre-installed cables attaches on the side of the drive cage.  You may want to remove those cables from the clip to prevent any accidental damage to motherboard SATA ports and to allow the drive cage to freely detach.  Each drive cage can be removed by unscrew 2 screws of #1 in first picture below. 
I want to add one or two additional (3.5in) hard disks to my existing desktop, while leaving the original in place.  The top side bay with the original disk (labeled HDD1) mounts the drive into place with a plastic caddy within a metal bracket, and I have extra plastic caddies.  The additional bottom bays (HDD2 and HDD3) do not seem to use any such caddy bracket and are in an awkward place where I cannot simply screw into the disks' built-in screw holes, nor can I seem to remove the metal brackets themselves.  How can I go about securely mounting an additional drive here, without removing everything including the motherboard?
XPS 8930, mounting for HDD2 and 3?