On Tuesday, April 14, 2015 at 9:28:26 PM UTC-4, John Carlyle wrote:
Greg, yes I did download it again. I tried the upgrade a few minutes ago (in the dark) and saw no evidence that the USB drive was being read (no LED blinks on the PF).

-John, Q3

I've found I have 100% success when I disconnect the Butterfly Display, then power up with usb in, and 0% when it is connected (mine is an early core, 169). Once it is done, I reconnect and check it went through. I was a beta tester for 2.7 and it was suggested as a good idea. Adds a couple of seconds but has proven to be a good idea - for me at least.
USB 2 less than 32 GB is required for the stick.
Dan