I’m wondering how many other users are having this problem. Also, is this an appropriate topic for this forum?
After postponing it for a couple of days (“Ask Tomorrow”), I told the App Store Software Update dialog to go ahead and install the High Sierra security update on my 2013 MacBook Pro. It failed, leaving my SSD unable to find its Startup Disk.
After some research, I started it up while holding down ⌘+R, and selected the “Disk Utility” option…and learned the drive status was “unbootable”, so I opted to reinstall the OS (which was time-consuming, but successful).