Sounds like I lost the com processor reading some of the other posts on this topic.
the upload fails but the loop back test passed ok, no tx or rx leds, Uno runs my last skecth ok and blinks the led on reset
Any hope of salvaging this UNO r3 or is it ready for the trash can?
windows 7
UNO re3
loop back test passed
upload massage:
avrdude: stk500_getsync(): not in sync: resp=0x30
Got it working!! 
the Com port had changed from com3 to com4, all is well!
hi,
I'm having the same problem...how did you fix it?
Here is something that may be related to this. Go into the "Control Panel > Hardware >
System > Device manager", and click on "View > Show hidden devices".
What you will probably see is a list of COM ports which aren't even connected to the
computer, but had been connected some time in the past. [B.Gates never seems to
forget, and rarely does any garbage-connection either, it would seem - similar for the
infamous Temp folder under Local Settings].
On my 7-YO notebook, the first time I did this, there were maybe 50 items shown
there. The best I can figure is, every time you plug a device into a "different" USB port
than the previous time, Windose assigns a new COM# to it, and adds one more item to
the list, and which is never removed.
I now have labels on my USB hub ports so I always plug the same device into the same
port.
I changed my cable and then it worked no problem.