I had to erase an external hard drive, a WD My Book, because I had to return it due to defects. So I searched on the web how to do that on a Synology DS916+ NAS, but I could not easily find the solution. Therefore I did a more generic search how to do it under Linux, and came across the tool “shred” which I had used years ago for the last time.
I checked on my NAS, and the tool was readily available. So I ran the following command to securely erase the (external) hard drive:
Hope this helps people who need to accomplish the same.