If the television isn't detecting any USB device at all, it may have either been receiving too much power at the port (depending on the specs of the external HDD), or the port itself may not be working correctly.
You might try to reset your TV, and see if that helps. To do this, you can (with the TV powered ON), press and hold the EXIT button on the remote until you're prompted to "RESET?" Choose "YES" and [Enter]. Then try the port again. (Note - this will delete your video and audio settings)
If it still doesn't work, you may want to call the Samsung customer service center and have your unit inspected.
Ya, I recently bought a WD external HDD to hook up to the USB port on the tv and watch movies and what not from. Everything was working fine except I noticed sound would gradually go out of sync, then I would have to pause Media Play for a few seconds, resume play and everything was back on sync. So I checked Samsung.ca for a firmware update that might address this issue, I saw a new update so I decided to update the firmware, that process all went through fine, however did not fix the sync problem. Well now after a few days of using the external HDD connect to the TV out of the blue I went to watch something today and the TV doesn't recognize anything plugged into the USB port anymore. Not my external drive, nor any other freshly FAT32 formatted USB key. Any help would be great, thanks.