Michael Anissimov has done a few calculations to estimate the total information content of our entire species memory, past and present. This is based on Bell Labs research which shows we retain about 2 bits per second, regardless of the type of infomration. By that measure, 30 years of memories at 15 waking hours a day would require only 150MB to store. Hard to imagine that my whole life could fit on a thumb drive.
Finally though I have a scientific explanation for why I can't remember where I left my keys.