Okay, so here's the details of my disappointment:
I downloaded the two versions of Build 796, the fmt version available at the wiki of this forum, and the one that came directly from Bookeen through
http://www.bookeen.com/support/ebook-firmware.aspx
I tried each. Put one on the SD card, placed it in, switched on, then clicked the button for a long time.
It still hanged. I put the other one, repeated the proces.
It still hanged.
My understanding of this is that it hangs up somewhere before anything really happens on the device.
I understand the next step is to contact Bookeen. I will.
But does anyone suggest anything else I should try before that?
Thanks all.
PS, for the person who asked. I use Ubuntu, the Cybook Gen3 was quite happy with it.