As far as I know before removing the main HDD you have to remove the 2nd HDD which is placed near the CD/DVD drive.
After removing the HDD cover at the bottom of the unit, you can remove the 2nd HDD by pulling the tab attached to the HDD.
To remove the main HDD, you have to remove the HDD board firstly.
The HDD board is secured by one screw.
After the screw has been removed, you should be able to remove the HDD board.
Then remove the main HDD by pulling the tab attached to the HDD.
But be very very careful… and don’t use force in any case.