Norton killed my network! Long live Microsoft!

YES! The source of the problem was an unannounced update
to Norton Internet Security. After disabling Norton's
firewall and re-booting all PCs a couple of time, I was
able to re-establish network drives across the Linksys-
based LAN and MN-700-based sub-LAN.

It not your fault Microsoft!!! It was that abomination
known as Norton Internet Security that came "free" with
my motherboard. Just download/purchase the Anti-Virus and
leave the firewall alone if you want a hassle-free home
network.