Prevent Sierra from opening a Window for a new Volume
I haven't relied on this in the Sierra times, but you might get lucky with the venerable unix fstab functionality and removing your default commands that weren't effective:

diskutil info /Volumes/Web\ Connection


Get the UUID and add a line to your /etc/fstab file

UUID=12345678-1ABC-1CDE-1FGH-123456789012 none hfs rw,noauto


There are some more details at https://discussions.apple.com/docs/DOC-7942 covering a nice use of the vifs command which safely edits the /etc/fstab file in place. Some good discussion on the mounting mechanics are at: How does Mac OS X decide which volumes to mount? One of my disks doesn&#39;t mount at boot