I would like to confirm the post from Aiekakiku (thanks!) Errors in Due board/schematic files - #12 by Aiekakiku - Arduino Due - Arduino Forum
R23 is the wrong value. It should be 1K
On my DUE it was preventing the 16U2 from resetting the SAM3X and causing the "No device found on COMxx" errors.
It also prevented the SAM3X from being reset after successfully loading a sketch.
I suspect that many others have this same problem on the DUE.
my related post with details: Due appears as COM6 port but gives error "No device found on COM6" on upload - #9 by system - Arduino Due - Arduino Forum
Update: See also Due appears as COM6 port but gives error "No device found on COM6" on upload - #11 by system - Arduino Due - Arduino Forum for the necessary 16U2 firmware fix
Andrew