if you heavily use the usb ports on your computer, and i most certainly have on my 4 month old compaq, the contacts are likely a little worn or squashed down. the data signal contact is usually the culprit that isnt making good contact and can be 'tweaked' back to shape by carefully pulling up on it with a dental pick, but do this with the power off so you don't short out the port. a much more common problem now that everything uses those ports now.
Ray, September 2008
