I expected it to work without being connected too (and was assuming the USB-GND jumper was to avoid USB noise) !! (Would you be able to share a schematic by any chance ? Happy if it’s via PM and I wont share it - also happy if you dont want to). I just thought I’d try it as your image on the wiki has it connected and I couldnt think of anything else (when connecting over DFU I had been powering via USB and had connected the GND). It wouldnt connect without it jumpered and works fine every time with it jumpered. I still have 5v DC not-jumpered....
Yes, the goal was to avoid noise. But from you experience it seems to be required for Data (i still have to try 
)
Keep the 5V un-jumpered if you use a DCIn power, that avoids noise and also avoid 5V conflict (from the 7805 and from the USB) !
Sorry, I won't share the full schema for the moment.
Here is the power/jumper part.
Thanks Xavier - that’s really really helpful - I’m sure then that the GND jumper needs to be placed for that schematic... (I’ll leave the +5v jumper off !)
