MHDDFS is a fuse based driver. It is used to seamlessly combine several directories or mount points into a single virtual file system. Since it is not in the Linux kernel it uses a significant amount of processing power and suffers somewhat from reduced write performance. If that is not acceptable try to Install AuFS. For home server purposes MHDDFS is perfectly acceptable and simple to set up. There are only a handful of options of which a few are essential for basic customization.