Be reasonable, he's trying to help you. Detecting the presence of power is no guarantee that the PC is running the application you want. Also the buffer doesn't fill up just because there is no program at the other end reading it.
You haven't answered the reasonable question: why not send a message to the Arduino to tell it so start transmitting? That is the reliable way. Of course you need to send another message to tell it to stop later.
PaulS has a high post count because he helps a lot of people. Some aren't very helpful back. For example, they ignore his questions.
Regarding your question, "why not send a message to the Arduino". Honest, I was sure I said enough to explain it, well, if not, here is my more detailed explanation to you:
I have a program that receives data from COM port (without handshake) and "handles" input stream in a special way - it displays received numbers as a long diagram, that is, ECG (not necessarily, it might be EEG, or EMG, or what not), applies different filtration and peak detection algorithms and many more. 'Handles' means 'it does a lot of things with', is it more clear that way?
I can configure input port parameters - port number, speed etc, but I cannot modify program code to generate any "data transfer request", this program acts as a passive receiver.
I am using arduino board for interfacing different sensors with that program; it is also important to provide a possibility for Ard board to accumulate data in standalone mode, without connecting to PC. So if my code could determine that there is something , anything connected to USB port, then it would transmit sensor data; if not - data is logged on SD card.
I'm afraid I can't explain this obvious requirement more clear; besides, this is probably not that important. Point is, I am relatively new to this particular board and I had some sort of illusion there might be some hidden tricks I am not aware of.