the main issue here is that the maximum external memory ( here sd ) supported by this cam is 2048kb or 2 gb.
if u use an 8 gb card ,it may detect the card but there are possibilities that the entire system may hang or your camera may use only 2 gb of available 8gb/
you can test this. borrow a card from the vendor from whom u bought the 8gb card , preferably another 4 gb or 8gb card, and try.im quite sure that the cam will show the same locked screen.