There appears to be a problem with your HD. Whats probalby happening is the data has 'popped down' for lack of a better term.

You see, when you save data, the arm in the HD takes itself and makes an imprint on the disk, depending on what is supposed to be stoerd there. Sometimes, when the hd is winding up to read something, the arm in your HD may hit one o those bumps and make it deeper, so deep that it can't be read anymore, or it doesnt register as what it should.

Your solution is to go on a science website and purchse a very powerful magnet. Open your case and put the magnet on the hard drive, and it should pull those grooves right back up.Problem fixed.

I wish I knew about this when my laptop started acting up. now you know.