Synology's DSM interface doesn't give you a GUI option to securely erase an attached USB drive. So this procedure documents some shell commands you can do on the Synology, to overwrite the entire disk partition with zeroes.
In this step by step guide I will show you how to secure erase your disks on a Synology NAS device. Warning: Before you perform a Secure Erase procedure on your disks, make sure your Synology NAS device is connected to an UPS (Uninterruptible Power Supply