avrdude: stk500_getsync(): not in sync: resp=0x00

I am getting the avrdude: stk500_getsync(): not in sync: resp=0x00 error.

I just bought a second uno r3 board. I plugged it in and it is assigned to com4. My first Uno is assigned to com3. I checked the drivers on com4 and they are fine. when I upload the sketch I have the correct port and board selected and I am not using RX0 on my project but for some reason I am still getting this sync error. Does it have to do with having both board plugged in? I have tried uploading the sketch with just the new board plugged in and with both. I can still up load sketches to the original board on com3 and it works fine. I am at a loss at this point.

Incidentally the project on the new board is the zoetrope beginner project. Any advice would be appreciated. thanks

That's strange...did you try to press the reset button right before uploading the sketch (press the upload sketch button, wait for it to compile then press reset quickly) ? You shouldn't have to do this on a regular uno board but that usually fixes the not in sync errors.

Just tried the rest button but no joy. thanks for the suggestion though.

Hi, I got one arduino UNO board, one USB cable and one usbtiny programmer.
I got the same error:
avrdude: stk500_getsync(): not in sync: resp=0x00

but the cause/situation is different. Post here for a discussion.
And I've already looked up some ways, still not fix it.
The program I am using is the blink one in the example folder, very easy.

(1)after I choose the correct board type, serial COM port, I still got this error.
So I think it does not relate to these settings.

(2)I tried the following ways:
burn boot loader by usbtiny programmer.
Arduino IDE: Tools -> "board", "serial port", "programmer", then, "burn bootloader".
or
disconnect all connections, then, reconnect them.

So, I think this issue does not relate to bootloader.

(3) But I can upload the program by usbtiny programmer.
I think it means my arduino board works fine.

Can someone give me a hand? Or any helpful discussion.
Thanks in advance.

One extra question:
(1) ISP(in system programming) is available for AVR chip. And one ISP programmer is required for this feature.
I remembered once I can upload program only via the USB cable(I mean, not through the usbtiny programmer). I think this is the ISP way.
But why I did not see any ISP programmer or circuit on arduino UNO schematic?

Thanks very much.