Further info on this: The DNS-343 fell off the network yesterday while I was copying the contents of a 2TB 3.5" disk from a docking station on my laptop to a 2TB JBOD disk in the NAS, so the copy failed. After I spent several hours researching and attempting to get it back, I gave up. Then magically late last evening I logged into my WIN 10 machine and all four of the mapped drives are back. During the interim and through this, all our outher network devices, including several computers and another NAS device also with four drives, functioned normally even through several complete network restarts.
I have one more thing to attempt which I learned from the other NAS device (different brand). I am going to remove the DNS-343 from the surge protector it is plugged into. The other NAS several times reported a 'Power Supply Failure'' which magically went away when I took the surge protector out of the setup. This is a risk, but so is the unexplained failure of the DNS-343 to appear when needed.
Even though this DNS-343 is only a couple years old, since it is unsupported (didn't know that when I bought it), I will probably be replacing it shortly, but not with a D-Link device. By comparison, I'm also replacing a desktop computer which also faild but only after a 14-year lifespan.