Most likely the plugs have heated up and cooled down to expand and constrast. (maybee even melted some). All I can say is at times it really can take a lot of pressure to get the plugs undone. Might want to try a hair drier to heat it up and see if u can get it to slide off after its become a bit more malable. You do run the risk of course damaging the components but it seems like at this point they are brining no value but instead create pain because you can't get them out.
Yeah I'll try with some pliers. I was just scared of breaking it. Thanks for the info.
The plastic part of the connector is pretty tough. Use the pliers to grab that, not the wire. Try to rock the connector side to side as you pull directly away from the drive.
I've had some 4-pin Molex connectors that seemed to be welded to the drive, but some careful work with pliers saved the day.