I’m speaking from some pretty rusty experience, but I believe that different PS3 fat models use the same types of hard drives (2.5” SATA) but in different orientations. Some versions of the console have the drive slide in with the connectors facing the direction of insertion, while others have the drive slide in with the connector on the side. For the later, you have to slide the drive in, then over to get it to engage. The drive is supposed to mount in a caddy, which slides into the system. If the caddy is missing (which I’m guessing yours is), it’s difficult but not impossible to get a drive into one of the sideways-mounted systems. I’ve used some folded up paper to hold the drive in place once I’ve finessed it into the connector.