I'd run chkdsk and let it check for and attempt repair of any bad sectors it finds. Bear in mind that depending on the size of the drive, it could run for several hours.
I have an external hard drive that I use for back-up of all my desktop files. Now, when I turn on the external drive and go into "Explore" in Windows Vista to drag & drop files, "Explore" locks up. If I turn off the external drive or unplug the USB connection, "Explore" then is operational again. I wonder what is causing this problem all of a sudden and how to fix?