A hard drive is a lot like a stack of re-writable compact discs or DVDs. They call the 'DVDs' platters. Plenty of hard drives have multiple platters within. As suggested, one of the platters was likely damaged. It's lucky that it worked out as it did, I'd be surprised if it would work at all after being dropped. Personally, I'd back up any data from the drive that you need and discard the drive inside the external hard drive.
Warning, this is a bit of a rant, please feel free to ignore this. External hard drives are almost all coffins for data. Much like you just experienced, most hard drives weren't designed to be jostled in such a manner and people might go about with them like they were a flash drive. The very worst aspect of external hard drives are the lack of proper cooling. Many external hard drive enclosures have a single tiny fan that moves very little air, and plenty have no fan at all. Heat will destroy anything, computer components are pretty sensitive to heat. This is why I recommend avoiding external hard drives if possible. If you absolutely must use an external hard drive, I recommend making the enclosure yourself. Just as an example, with the use of http://www.newegg.com/Product/Product.aspx?Item=N82E16812232002&nm_mc=TEMC-RMA-Approvel&cm_mmc=TEMC-RMA-Approvel-_-Content-_-text-_- something like this, you can rig up your own box with a 80-120mm fan that will keep your drive happy and cool all the time.