Take the front end of the drive and look for a plastic tab about
3cm wide on the H bracket.  Press that down away from the
drive and the metal plate will slip forward and out upwardly a bit.
It can take a fair bit of pressure to unlatch and slide it out of the
H bracket, and may requrire 3 hands to keep from dropping things.
In a pinch, you can just drill holes for the underneath drive and
put the underneath drive on with a set of flush-head screws.
Use the metal plate drive on top.  I did that on one machine
for mounting the second drive and for mounting the cdrom
(anyone got a spare cdrom plate and front bezel?)

Bob Keys