It's probable that I had never rebooted the VM after the one re-boot associated with the re-installation of GuestAdditions. After I re-ran the GuestAdditions file, all was well – I thought. I recently (in the last couple of days) updated Win10 to 1703 (the Creator Version or whatever it's called) and after that update I recall searching the VB forums because the VM guest could not find the bridged folders on the host. I did see this somewhat related post ( viewtopic.php?f=6&t=62874&start=15), but it doesn't seem to be the same thing.
After re-booting the Mac on which VB is running, I launched VB and tried to go back to this Windows 10 VM. I just saved a snapshot and shut down VB. Cannot attach medium '/Applications/VirtualBox.app/Contents/MacOS/VBoxGuestAdditions.iso'
