That sounds suspiciously like what my external hard drive did when it quit. It had a Samsung hard drive in it, but that's just about exactly what mine did. With relatively little warning, it started reading and writing very slowly, and then essentially quit altogether leaving me with pretty much the same symptoms that you are describing.
I took the drive out of the external enclosure and attempted to connect it internally as a secondary drive in my PC itself, but sadly I still have a hard drive that just makes funny noises and lots of data on it.
Dave