I don't know the Pi 2540 from the inside, therefore I can't tell you what has to be done in detail, but in general you have to remove the screws that fix the HDD case (is itreally only one?) and then pull the HDD (including case) apart from the connector.
In my case it looks like this:
https://www.amilo-forum.de/download/file.php?id=3035I have to remove three screws and the pull the case to the left.
Can you make a photo please?