Synology to enforce use of validated disks in its NAS. And guess what? Only its own disks exceed 4TB : r/synology
DS1621+ shows compatibility with 16TB and 12TB of Exos drive; should 10TB from same series be compatible, given it is for many other DiskStations? : r/synology
Best Synology DS218+ compatible hard drives | Windows Central
Refurbished: WP Arsenal 12TB SAS HDD 7200RPM 3.5-Inch DAS Hard Drive Compatible in NetApp, SuperMicro, Synology, JBOD Storage Expansion Enclosures - Newegg.com