Have been bumping into a strange issue regarding the COM connection with the Uno. Serial communication works perfectly until I ground the Uno ground to a 24V switching supply ground which is not powering the Uno. I found out that when i switch the supply on or off after once or twice, the Uno will only be able to receive serials from PC but fail to send anything back to the PC. Of course, if i pull the cable and plug in the USB cable again with the 24V on, everything is ok as well. It seems to be that the common ground is causing some issues as i turn on and off the PS. Furthermore, if i do not connect the PS ground to the Uno ground, things are ok as well. FYI, the 24V is a power source for a stepper motor drive. Thanks guys.
So it sounds like your 24vdc switching power supply has three output terminals, +, - , and 'ground'. If that is the case then you have no need to wire both the - and 'ground' to a arduino ground pin, just the - terminal needs to be wired to the arduino ground. Your switching supply probably has a three prong AC connector? And if so you are probably creating a AC 'ground loop' that your PC's USB buss does not like at all, which is effecting the serial communications. The Power supply ground terminal might be a safety feature to allow a path for fault currents, but at 24vdc that is not a requirement to utilize in my opinion.
Anyway those are my thoughts on your symptoms. A link to your specific power supply and a drawing of how you have everything wired up (arduino, external PS, stepper motor/control wiring) would better help us to verify where the problem might lie.