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Seems that somewhere along the line Microsoft removed SMB1 access from Windows 10 Home.
But my C1200 modem requires SMB1 protocol for access to the USB-attached disk.
After chasing Microsoft links all over the place to no effect, I asked Google and the first link shows how to install SMB1!
Basically: at a Command Prompt, type optionalfeatures and select SMB1.0 Client from the list!