Synology DiskStation DS620slim 6-Bay NAS Enclosure - Manual (Page 9)

9 Chapter 2: Hardware Setup
3 Insert the loaded drive tray into the empty drive bay.
Important: Make sure the drive tray is completely inserted. Otherwise, the drive might not function properly.
4 Repeat the steps above to install all prepared drives.
5 Drives are numbered as illustrated below.
Note: If you want to create a RAID volume, we recommend that all installed drives are of the same size to make the
best use of drive capacity.