I'm having a bit of trouble getting the plastic drive bay button things off though, look like they can only be broken off?
If your patient you can line up the drive caddie mounts and secure them in, but it's a lot of messing about and they really aren't as secure as they should be. If you pinch it above and below the button you can remove it, but the spring mechanism beneath is pretty much impossible to replace once you do. Honestly I just rip them all off and use screws to fix the drives in place as it looks neater and is far more secure.
ScanProAudio & 3XS Audio Systems