I have two new UNO r3s from different suppliers. Ostensibly both genuine. One worked straight out of the box the other, I discovered, had an unprogrammed 16U2 and no bootloader in the 328. In Win 7 (64) I loaded drivers and hex file ...\atmega16u2-Uno-Rev3 into the 16U2, and replaced the 328 with one loaded with Optiboot\Blink. This board now passes the loop-back test but will not upload. I use Blink as a test upload.
When I upload to the working Uno, I see furious flashing of both TX and RX but with the non-working Uno the TX remains blank and after some seconds I get the error avrdude:stk500_getsync()... etc. In this test I simply connect the USB cable first to one board then to the other. The working Uno comes up on COM5 and non-working on COM3.
Seems to me my problem is with the 16U2 (or is it a fault on the board?) I'm new to this stuff. Have I missed something obvious? Thanks.