thanks for fast response,
i wrote in MS visual 2010 C++ , using virtual com port , with your codes from a prev post :https://forum.pololu.com/t/maestro-c-serial-communication/3181/5 
I’m currently out of the workplace which my maestro is at. But i will try to recall what i can remember. Earliest i can verify again will be on coming Monday.
Its running on win xp SP3, i have not checked the device manager before, but i can remember there is no “disconnected USB” sound when this issue happened. Only when i physically re-plugin the usb, the “disconnected USB” sounds followed by “UBS device detected” sound.
I cannot verify the current state now, but if i can remember, i did checked “Never sleep (ignore USB suspend)”, applied the settings, and unchecked the device manager->(all) USB Root Host-> power management settings.
The Yellow LED was lighted, but i cant remember it in a flashing state or lighted state. The green LED was off. No red LED too.
No error message on control center, until an exception when try to send commands from the slider bars.
When running my program, with this bad state, the printed message was “Error: Unable to write Set Target command to serial port. Error code… (sorry, i cant remember)” , which means the writeFile() from the “maestroSetTarget()” had returned false.
i hope i can programmatically resolve the issue. maybe by making use of the error message detected. i don’t mind if i have to add additional hardware to sustain the maestro, or do something about the fan. Any suggestions would be appreciated.
PooHee