OK, I got the guide for it... DX-PHD35
From the figures it looks like an extruded aluminum case either has two halves or four parts that are snapped together. I don't see any means to cut the case off without the risk of damaging the HD (very little clearance).
From the looks of it you have two screws on each end to hold the plastic or metal end covers. Which I suspect you have already removed - Correct?
You then have two side rails which the HD is mounted to and a circuit card inline with the HD between the rails that is part of on of the end covers, the other cover is just loose.
So the only place I can see a problem here is if there is a lip on the large flat plate (top or bottom) that is snagging the edge of the drive.
Can you move the drive at all? In and out even a little bit.
And can you place a piece of thin stiff cardboard or plastic between the drive and the top/bottom covers to see if that is where you are hung up on?
If you can get a good picture at the exposed end (the side with no ports) straight on (focus set for the edge surface) and the large side at an angle (like figure 5). Maybe the way the case was built can give us a clue on how to break it apart.
OK, I think I see what has happened. Yes, you are correct on one side you missed the slot for the rail. I think it might be possible to save the case here and the drive!
In this model they used a single extruded case so we can't snap it apart. But, what we can do here is distort it so we can enlarge the gap the drive is stuck in. I won't bore you with the math on how this works, its just about changing the inside area just enough so the inner assembly can slip out.
OK, here's what you'll need to do. First get some masking tape to cover the outside case so you don't scratch it up. Hopefully you have a friend who has a good bench vise. Using some hardwood blocks place them across the larger sides of the case one on each side. Then place this sandwich into the vise with the jaws pressing on the wood.
The trick here is as you squeeze the case the case ends will push outward slightly (go slow here) at some point the space between the walls will lengthen just enough to allow you to slide the assembly out. You could try measuring it with a fine pitched ruler, you only need a few millimeters).
You do have one small issue here as there is a screw that is also rubbing on the case bottom (near the power outlet). You may find it is jammed up using a screw driver you may need to pry the edge of the screw so it clears the lip its caught on.
If all goes well you should be able to re-use the case (this time making sure to get the rails into the slots). - Good Luck!