Synology NAS HD6500 Hardware Installation Manual page 13

2 Pull the drive tray out by its handle as shown below.
3 Install 3.5" drives: Place the drive in the drive tray.
4 Insert the loaded drive tray into the empty drive bay.
Note:
Make sure that the drive tray is completely inserted. Otherwise, the drive might not function properly.
5 Push the handle inward to secure the drive tray.
6 Repeat the steps above to install the other drives you have prepared.
13 Chapter 2: Hardware Setup