Hi Ibrahim Dachlan
you may try opening up and check as shown on the video link below.
https://www.youtube.com/watch?v=hRIrtNK74zk
https://www.youtube.com/watch?v=jL8RkDuI84E
as per user feedback on youtube by Günter Höller:
in summary there are 2 different models on market of this. one is upgradable with an already built-in sata 2.5" disk (different bottom chassis of keyboard unit), the other model isn't upgradable.