My solution turned out simpler. It was the Wi-Fi adapter! I thought windows 10 did auto updates. I went to the vendor website and they said they didn't have a windows 10 driver. I swapped...

Ditto.



My Windows 7 box hosting the shared folder (Call it (A)) is connected by cable, the rest are wi-fi.



That's right, I can see it and the shared folders in file explorer on the win...