insert pin at "B", and move pin toward to buttom gently to release latch. This will move plastic block (colored in Cyan) behind front cover.(source: http://www.readynas.com/forum/viewtopic.php?f=20&t=5286)
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After that I removed the harddisk from the tray and used a spray on the latch and so far it is better than before.
The spray I used was Instant Cleaner from Philips which is used to clean off fingerprints and so on from electronic equipment.