To Ray’s question and suggestion: Yes, I have tried doubleclicking the .exe file. Doing so opens a dialog box with options Extract All, Run, Cancel: If I do “Extract All:, it tells me that all the files already exist (and they do). “Run” opens the User Account Control dialog, so I know something is happening; but when I hit Allow, the file closes immediately. This is the case whether OS has been just plugged in and is still showing “connecting…”, has been plugged in long enough to be showing “MC_________________”, or whether I held down B2 for less than 2 seconds and the display is showing “Reset all?”
To TechFan’s suggestion: I removed all old copies of the .zip file, then downloaded a fresh copy to Desktop and renamed it osUpdate.zip. The osFirmwareUpdater file I cannot rename (not an option under Properties). Doubleclicking the osFirmwareUpdater.exe file has the same effect described above; i.r., Run opens User Account Control, but the file closes immediately when allowed to run.