Synology's DiskStation DS715 borrows an interesting feature from the DS1515 that we recently reviewed. Look on the rear panel of this dual bay NAS and you will see that in addition to the dual Gigabit Ethernet and two USB 3.0 ports you also get an eSATA connector. This port can be used to connect a DX513 expansion unit (which costs £340) that can house an additional five hard drives. In other words this dual bay NAS can be transformed into a seven bay NAS which has a huge effect on the maximum capacity and also means you can explore the options of RAID 5 with a parity drive to protect your data, or you can divide the drives into a number of arrays. You get many more options with seven drives than you do with two but while this is interesting we don't have the DX513 to hand and are considering the DS715 purely as a dual bay NAS.