The device in question is a Synology DiskStation DS213J (NAS). I know I can encrypt shared folders, but unfortunately not the home/homes folder. All I want is that if my NAS is stolen than all data is unusable.
I don't need to have different encryption for each user, just a collective one which I manage. I know that TrueCrypt and (I think) Windows with BitLocker can encrypt the disk before you enter the OS. I was thinking the OS of my DiskStation NAS could do something similar.