I have a theory that when I tell others I always catch a lot of grief. It came about this way:
Up until I was in my mid forties, I was able to remember everything no matter how trivial. It was like a photographic memory but not that intense. Everything I had ever read or seen was etched in my mind and I was able to recall it with remarkable accuracy.
Then one day (and I remember it vividly), I was unable to recall the punch line to a old joke I was being told. I was stunned. The only explanation that I could come up with, at the time, was that my brain had filled up and that in order to make room for new knowledge to "stick," I had to delete something. My mind, then, when given new data would automatically go through all my memories and delete the least likely to be needed again or that which could be easily looked up... in order to "make room."
Every time I tell this (and I still believe in it today), I get not just laughed at but ridiculed for believing such nonsense.
Of course, the fact that it mirrors the behavior of computers -- disk drives fill up regularly... and unexpectedly -- makes it much more difficulty (for me) to discount.
Anyway, since that time, I have been content knowing that I can remember exactly where I can get the data I am trying to remember. An example is that I can remember exactly where an individual image, of 10s of thousands, which are spread over ten external hard drives can be found. Or in which of my many books a particular quote is located.