So, after much heartache trying to make an “acceptable” sd card, I’ve found out it wasn’t the SD card’s fault.
What I saw:
During boot the Nvidia logo would appear and then everything would go dark. No obvious activity of any kind. I could not ping the enet; there was no connection light on the enet. Nothing.
I bought a new jetson nano, which behaved the same way out of the box. So either both are bricked or there is something else.
It turns out that my – till now – fabulous brilliant and utterly wonderful in all ways 5V supply was not putting out sufficient voltage. I still have to find out why. A 2nd supply was putting out 5.34V (as measured from the GPIO pins) and that worked. My original supply appears to be putting out 5.04V. My surmise is that I have finally plugged in too many devices to my 5V supply, though proving that will take some doing (to position the power meter).