Synology DiskStation DS413j Quick Installation Manual page 8

6 The disks' positions are numbered as shown below.
 For 3.5" Hard Disk (only): Secure the hard disk trays with the screws (for securing hard disk trays)
provided, which is not required for 2.5" hard disk.
7 Place the upper case back to its position.
8 Chapter 2: Hardware Setup