I find that needle nose pliers will help as the heads of the screws are not as flush against the bottom (or sides depending if you have a slim or fat PS3). as for replacing the screws dunno what you can do, the drive fits pretty snug in the caddy and once it is 'plugged into' the console it is not gonna fall out of the plug it connects to just do not bump or jostle the system after you do the swap. I am pretty sure if you call PS3 customer service you MAY be able to order replacement hard drive caddy screws, good luck
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EDIT: Sony does not carry replacement screws or the caddy. but if you take one of the 4 screws to a home depot or any store that sells loose screws they may be able to match it, or see if any PC repair shops have any as they resemble the screws that hold PC hard drives and optical drives inside the PC towers.