I ran XP chkdsk overnight and it completed this morning without errors or bad sectors. Amazingly, the clicking sound is gone for now (touch wood). I moved the PC and changed the USB port, and also tightened the power cord. I have a feeling the power cord is a bit loose, thus not supplying enough power to the unit (read on Newegg that for the internal version of this Seagate drive, when not enough power on the 12V rail causes the drive to click). However, I am leaving the USB port the way it is. Not going to experiment with it.