I have 4 Arduinos, and 2 pcs.
One of the Arduino may be faulty.
One is working and can be programmed. As this is my alarm clock, it is now only done from my laptop.
The other two just won't flash from either.
One of those has been tried on a third pc and also fails.
The "faulty" one works in that I can upload the BLINK sketch and it works.
Other sketches upload but as some of the I/O lines are suspect, may not work.
Suffice to say the LED stops blinking when the new sketch is uploaded. Then starts blinking when the BLINK sketch is uploaded.
Also there are no errors.
The two new cards give this "not in sync" error.
One is a freetronics one and is a genuine Arduino uno.